London with 9 and 12 year olds - activity suggestions, and what needs to be booked ahead?

The London Transportation Museum in Covent Garden is terrific.

The Tower of London.
Not sure the changing of the guard is really worth it, plus the wait time.
Have a great trip,

Greenwich. You can get a boat there. Various things to see that may interest them.

Kids like the London Eye and the walk along the South Bank is always - street entertainers etc. Go as far as Tate Modern and have a look inside. It’s free.

Chinatown is fun for eating.

Science Museum has a hands on exhibit that you have to pay for. The main museum is free.

If you have girls they may be interested in the Hello Kitty pop up cafe at Somerset House. I know mine are desperate to go.

Theatre! The Play That Goes Wrong is funny. Matilda obviously is great for families. Have a look and see if anything else looks good.

I haven’t been but the ice cream parlour at Fortnum and Mason is meant to be good.

The Natural History Museum requires a pre-booked entry time (free).

The Science Museum had a lot of great kids activities when I visited (more than 10 years ago); I don’t know what their situation is now with regards to pre-booking.

London Walks has a Harry Potter film locations walk on Saturdays. I’ve taken lots of walks with London Walks but not this particular one. While you need to reserve, you can typically just do this the day before your walk.
https://www.walks.com/our-walks/harry-potter-film-locations-in-the-city/

Natural History Museum too, also the Young V&A https://www.vam.ac.uk/young (used to be the Museum of Childhood), and if your kids are interested in history, the Imperial War Museum. And the Zoo, of course.

Take them to the Museum of Natural History. Lots of neat things there for kids. There are some great theater shows for kids right now: Frozen, Lion King, Matilda, Shrek, Wicked and even Harry Potter and the Cursed Child ( Palace Theater) and more. I think Greenwich is a neat place for kids, the Cutty Sark, the Naval Museum, the Prime Meridian and combine it with a boat ride on the Thames on your way there or back. The Tower of London with its Beefeater led tours are funny and interesting. Do a daytrip out to Hampton Court or Windsor Castle ( both are relatively close by and can be reached by train). Stonehenge is a long day for little kids. I have not been but the London Transport Museum looks like a place kids would enjoy and there are always lots of street entertainers doing their thing all around that Covent Garden area. The Diana Memorial Playground is near Kensington Palace if you are looking for a place just to walk and play.

We went this summer; my boys were 10 and basically 8 at the time. I wrote a loooooong trip report, but here are the highlights.

Definitely visit the London Zoo, and especially the lemurs; you can walk through their enclosure and they come so close! We also really enjoyed the Tower of London. We got there about 20 minutes before they opened and were probably among the first 100 or so people to get in. I wish we had stayed longer here. We combined the Churchill War Rooms with Westminster Abbey on one day. Westminster has an excellent family audio guide that kept the boys engaged and entertained. We did a short trip to Greenwich (everything closed early for some reason the day we went) and took a boat back to London; the cruise back was nice, even in less-than-ideal weather. The Transport Museum is also, surprisingly, a lot of fun. You have to buy an annual pass, which at first I thought was annoying but really came in handy when we visited Covent Gardens again and needed to use the bathroom!

The British Museum was really crowded, to the point of it being impossible to navigate through some rooms. We went earlier in the day; maybe closer to closing would be better? Or the day it closes late? The Natural History Museum was interesting, but also crowded. Your kids would really enjoy the WonderLab in the Science Museum (Museum of Science? I forget the official name). You do pay to get in, but it was well worth it. It's also pretty well contained, you can sit in one place and let them roam around freely. If you got to Borough Market, go during off hours. We went lunchtime on Saturday and it was a mess.

For day trips, we went to Bath. They really liked the Roman baths, but after that there's not much to interest kids that age and I wish we had caught an earlier train back. I really wanted to go to York, but thought that would be too much for a day trip. We also considered Portsmouth, with all the ships. If you got on a tour it might be worth it to do Stonehenge, but I wouldn't want to navigate it on my own with kids. If you can't get into the Harry Potter Studios, Oxford might be a good substitute. I also second the idea to do the Harry Potter Tour with London Walks. We did their Hello London! Tour and enjoyed it.

I would say get as many advanced tickets as possible, even for places that are free. It saves you a lot of time in line.

Have a great trip!

These are not anything that needs to be pre-booked but most of the museums have special kids things like "activity packs" or the like. Check out their websites. The V&A, the Natural History Museum and the British Museum are all good. And all free, even if some you need timed entry pass. So you don't have to worry if everyone has had enough and just wants to leave in an hour.

Also google changing of "horse" guards rather than the "regular" changing of the guards. It's more accessible and it has horses!

And if you can't get to Harry Potter World consider London Walks. They have many walks that kids would enjoy and they have HP London locations walks. https://www.walks.com/our-walks/harry-potter-film-locations-in-the-city/

Finally, a good resource for all things for kids in London is the on line magazine Time Out London https://www.timeout.com/london/kids

Mudchute Farm
Tower Bridge Experience ( also check the lift schedule for the bridge )
Science Museum
Thames River Cruise
Ride the tube
Harrods Food Hall
Seven Dials Market
Museum of Water and Steam
Richmond Park to see the fallow deer
Olympic Park
Brick Lane Street Market
Shoreditch Street Art
London Zoo
Theatre, theatre, theatre

The Churchill War Rooms are a book-in-advance place. You could run into a really long line otherwise. Note that the war-rooms section is rather small and doesn't really take long to see. The time-consuming part is the Churchill Museum, which may not hold a great deal of interest for children the age of yours. If they are only interested in the war rooms, that's a pretty expensive ticket.

The Science Museum's exhibits are modern and I'm sure they would be appealing.

It's true that the British Museum is often really packed, especially the ground floor (which is where the ever-popular Egyptian collection, Elgin Marbles and Rosetta Stone are). Museum staff told me weekends, Fridays and holidays are the busiest times, and rainy days are worse than average. The museum is understandably popular with local families, so if the British kids are out of school the week of your trip, every day may be a weekend day from the crowd perspective. In my experience, you'll be in line for 30 minutes or so if you show up a bit after opening time without a pre-booked entry. Booking an online entry time should get you to the front of the unavoidable securty line. On the Friday I took advantage of the late hours, the museum did start to clear out a bit in the late afternoon (not as busy at 4 PM as at 10:30 AM or 2 PM), and it got quieter as the evening progressed. However, I was mostly upstairs and cannot tell you what conditions were like in the Egyptian Gallery. One issue with planning a late visit is that food service stops on the regular schedule, so you won't be able to pick up a snack at 5 PM or something like that. I don't know exactly what time the grab-and-go place closed, but it seemed really early to me.

From everything I've read, the regular changing of the guard ceremony is a real waste of time, requiring very early arrival to get into a spot from which you (especially short youngsters) can see anything. I don't think it makes any sense on a short visit when there are so many other things to do in London.

The usual suggestion for visiting the Tower of London is to go first thing in the morning and go straight to the Crown Jewels, then see the rest. The Yeoman Warder's Tour draws positive comments, but I haven't take it myself.

The London Eye is expensive but would probably be a big hit if none of you are afraid of heights.

The London Walks are really good. The guides are licensed and many are out-of-work actors, so they are ver engaging.

London Walks does a fantastic Harry Potter walk. I took it two years ago. They take you to some sites from the movies. I’m sure your kiddos would enjoy it. I know I did.

https://www.walks.com

What a great trip! I love London, have been there many times most recently last month, and lived there when I was 9 which is many years ago (if you want to drop your kids off at a good school and pick them up in a year, I have a recommendation, just kidding). So we don't have a lot to go on -- don't know if boys or girls, and for interests, we know they like Harry Potter but not necessarily traditional museums, but not much else. I'm guessing it's their first trip to Europe? So you want to accommodate their interests, but also stretch their horizons a bit, I would think. And you don't have a very long trip to do it. Some great advice already, to which I would add:
Just stick to London, there are many good day trips but more than enough in the metro area.
Hyde Park is fine, but it's just a park. At 9 and 12, your kids don't have unlimited stamina, but they aren't toddlers.

Forget Harrods. Many better places to shop if you want that.
Definitely the Tower of London! Jewels, uniformed Beefeaters with tales of gore and treachery, a 1,000 year old castle that's the heart of British history, do that and go early. You can't get anything like that at home.
You don't have to be religious to appreciate at least one Gothic cathedral, again, nothing like it on the West Coast. Westminster Abbey (technically not a cathedral), is great, but expensive, and your kids may not appreciate all the monarch's tombs. Go to Southwark Cathedral, much more economical, they just ask for a donation (which I recommend you give), and it's just across London Bridge and very convenient to Borough Market for a nice lunch (or the George Pub, very historic, but just don't go on a major rugby tournament day as service backs up!).
For an experience of Britain's amazing 20th century, while the Churchill War Rooms are good, also they are expensive, take them to the Imperial War Museum, it's free so no worries if they don't last long there -- and I've loved that place since I was 5.
Yes to a Thames River cruise, to Greenwich if you can (when there, maybe see the Cutty Sark clipper ship, but otherwise it doesn't sound like the major attractions fit your main interests, but still, a nice place to go and walk around and see the river).

British Museum is great, go in the afternoon when lines are shorter, get a "highlights" brochure and keep it short, Elgin Marbles, Egypt, Roman Britain, and (my favorite) Standard of Ur. But your time is short, if you don't get here, it's ok.

You may also enjoy going to Windsor Castle or Hampton Court Palace. If your children are girls they may enjoy going to tea. We took two grandchildren to London in August and highlights for them were the Tower of London, the London Eye, and evensong at Westminster Abbey. (Evensong is free.) The park by Kensington Palace is nice also.

This is all fabulous info - thank you! My older (12) is a girl, the younger (9) is a boy - so we need a bit of compromise from both! They are pretty good at working together, though. I think we will take the advice to stick to London, as we will take the chunnel to Paris for another 4 nights, and I've heard that is quite an ordeal these days.

The Chunnel is a name that only Americans seem to use these days for some reason. We would all refer to it by the operator's name, Eurostar or Eurotunnel if you were going to taker a car through.

I don't think it's particularly difficult to use the Eurostar these days but because of the extra immigration procedures following Brexit you are advised to be at St Pancras 90 minutes before your train departure.

Have you tried Golden Tours for Harry Potter Studio tickets? They have packages including transport from central London at quite an expense but are often useful if you have missed the standard tickets.

Yes, I've tried everything to get the Harry Potter Studio Tour tickets - Warner Bros web site, Golden Tours, Tripadvisor, GetYourGuide. No luck yet, sadly.

We took our two grandsons to London a few years ago. They were 12 and 9 at the time. They both loved the Tower of London (get there early and head to the crown jewels), Windsor Castle, and The Royal Mews. The Tower had a great workbook for kids, and the Castle and Mews had a wonderful audio tour for young people. The changing of the guard is difficult to see due to the crowds, but the soldiers amass at St. James Palace around 10 before marching to Buckingham Palace. Plan to get there early for a good spot on the side of the road. It includes soldiers, a band and horses. We got book ahead tickets for the London Eye so we wouldn't have to wait, and the boys enjoyed that even though it was drizzly. I believe Viator offers a couple of Harry Potter London locations tours. I took the 9 year old to the Transport museum in Covent Garden, and he loved it. We also went to the Lion King which was a big hit. (I think a trip to Oxford would be lost on young people the age of yours.)

If you're going to Westminster Abbey, it would be a good idea to find a video of the coronation of King Charles III. Point out Prince George and the other young people who served as pages. This may help to give your children a sense of perspective and see that young people were involved.

Have fun -- our trip was such a success that we repeated it three years later and adjusted activities accordingly.