London in October: Sights with Kids

Definitely the Tower of London for the boys, get there for opening in the morning to see it at its best. And if you’re lucky, you’ll see Tower Bridge opening too. If they’re interested in ships or WW2, the the Belfast cruiser is moored in the Thames opposites the Tower

If you want to do the Train/Ferry combo to Ireland there are two choices-
London Paddington to Rosslare (in the far south east) via Fishguard- that leaves London Paddington at 0748 (change at Swansea for Fishguard) and arrives at Rosslare at 1630.
You book that through the Great Western Trains website, not through Stena line (the shipping company)- the train arrives literally at dockside at Fishguard. The railway station is a very short walk outside the port. Through fares including the ship are £53.90 each (adult fares)- £269.50 for the family.
Or catch a train from London Euston at 0905 straight through to Holyhead, the ship arrives at Dublin at 1725. At Holyhead you are bussed from check in to the ship's berth in the outer harbour. That route is £51.90 each adult, ferry inclusive. (£259.50 for the family of 6). Book through the Avanti West Coast website, not Stena.
It's then a short bus ride from the port to the centre of Dublin from the port-small extra charge.

For castles, an obvious choice is Windsor castle. Two nice ones are Warwick and Leeds. Warwick has a Disney feel to it, but is a nice castle in its own right. Leeds Castle (Broomfield, Maidstone ME17 1PL) is very nice and can be combined as a day trip with Canterbury. In Ireland, Trim and Cahir castles were both nice.

How many days total do you have? I couldn't really tell from your post. Unless you have at least two weeks, my suggestion is to book your flights as Multi-City. Fly into London, sightsee there using public transport and short day trips to Harry Potter Studio, etc. Drop Stonehenge from this trip it is a major (and expensive) outlier. Get a flight from London to Dublin and then limit yourself to Dublin and Galway areas. Also easily done on public transport. Fly home from Dublin.

Thanks so much for suggestions thus far! Adding to itinerary. The castle is more about architecture and history, not interested in showy royalty, so does that affect suggestions? The train/ferry combo sounds like a fun trip and certainly different than what we experience here in Tennessee! I'll check that out.

If royalty isn’t of interest, then take Windsor off the list. I enjoyed both Warwick and Leeds about the same. Leeds has more of a manor house feel to it while Warwick a defensive structure. I did like the grounds of Leeds better. In Ireland I liked Cahir better than Trim, but Trim is closer to Dublin.

https://www.towerbridge.org.uk/

https://www.nhm.ac.uk/visit/exhibitions/titanosaur.html

Crystal Park Dinosaurs

By the way this happened to crop up in my feed this morning. It gives a good impression (albeit by a car driver) of what the Stena line ferries are like on the Holyhead to Dublin route-
This one- the Estrid and her sisters the Embla and the Edda are very nice Scandi-inspired ships. Other people have commented on other threads about being on ferries in bad weather. I have been on these in a Force 10, and they are as stable as anything, they cut through the seas like a cruise ship would. The only thing that stops them is if they can't berth at either end due to wind.
They are a far cry from the ferries of old.
Once when booking without train I upgraded all the way to a luxury cabin, which has it's own private deck space, the most glorious over sized port hole to curl up in and a complimentary mini bar. There are also standard day cabins (or night if on the overnight crossing).
When booking ferry only you can pre book meals with a big discount.
There is also an executive lounge which includes all kinds of nibbles, tea/coffee, soft drinks and alcoholic drinks, which is an upgrade if booked as a ferry only sailing.

https://www.youtube.com/watch?v=5ghk-OHrG3I

They might like all the tanks at the Imperial War Museum.

As far as Castles are concerned, the Tower of London would be top of my list along with Hampton Court. They definitely meet your criteria of 'architecture and history. Visit the Tower of London when it opens and head immediately for the Jewel house to see the crown jewels and then do on of the Beefeater tours.

Leeds castle looks stunning from the outside with the moat but the inside is the work of a very wealthy American in the C20th.

Warwick would be a long way for a day trip, so I would give this a miss unless you are planning to be in the area for a few nights. The family recently visited and their comment was Warwick Castle was expensive and they had to pay extras for parking. If you do visit Warwick, try and visit St Mary's church and especially the Beauchamp Chapel with the splendid tombs of Warwick castle.

Again, thanks for the various suggestions. We DO plan an open-jaw trip, flying into London or Dublin and back home from the other city. Any suggestions on which way to do it, London first, then Ireland is what I initially thought, but maybe the other way round is better? Total trip to be 12-13 days, divided more or less equally, depending on our final itinerary. Currently, I'm just putting down what we should (or want to) see. I'm not a big-city gal, so 3-4 days in a city and I'm done, but others may want longer in London. I always appreciate this forum and everyone's input!