Besides watching Rick Steves' episodes about York, you could search it on You Tube and come up with additional walking tour videos like this to give your family an idea of what is available. That way you may find if they're interested.
https://www.youtube.com/watch?v=O3WwXUFDlSI
London's must-see sights are whatever you all love the most, but one really easy (and high-value) one is the Treasure Room at the British Library. It's right near King's Cross station, where loads of the city buses go (and Platform 9 3/4 has a new gift shop). Plus the room itself is set up with low cases so you can see where your kids are, has many comfy benches all around the room for anyone who is getting tired, and has a zillion nifty historic things, including a headset with Beatles fan club records of them telling jokes. It's a great go-to place for rainy days later in your trip after you're getting tired out. :) Cafe' is right next door to the room, easy for lunch, cafeteria style.
I was a receptionist at Pax Lodge, the Girl Scout world centre in Hampstead, for 2 months, and the senior citizens were always SO grateful that I suggested that room to start their sightseeing off easily. If you have Girl Scouts in the family, Pax Lodge is on the 268 bus line, at the Rosslyn Hill bus stop, easy, and lots of activities for kids (check their calendar). It's a hostel that's open to everyone, don't have to be GS, but get a discount if you are. They have night programs sometimes & also sell discount tickets for attractions.
They're also located right next to Air Studios, where the music to Harry Potter, Kung Fu Panda, and loads more movies, was recorded. A former staff member from Pax Lodge that I worked with is just starting her new job there at the York train museum Tell Ruth we said Hi. :)
Probably just have the adults & older kids write a list of what they want to see in London, and go to the places that are on everyone's list. :)
Definitely stay in a London place as close to the sights you want to see as possible. Easy is way better with a city of this massive size. Plus with such a large group, you may be able to share a big taxi sometimes and save lots of bus or tube fares. There seem to be some good-rated hotels near Victoria Station, I search on Booking.com and then google the hotel name and go directly to their site, I think Syndey Hotel had really good ratings the last I looked at it. Victoria station is also where the Golden Coach leaves from to go to the Harry Potter studios tour, which can be a real bear to get to from other stations, it's outside of London in Watford Jct. Their bus is the easiest way to go, it seems.
Windsor's great and there's a Green Coach bus that goes from Victoria Station right to Windsor, then it travels on to Lego Land, also in Windsor, where they just added a load of new buildings from all over the world. That could be a good trip for a group with kids.
A friend of mine from high school lives over there, and he just took his 2 kids to Lego Land a couple of months ago, and they had a blast.
The site www.homeaway.co.uk is also popular there in the UK, in addition to airbnb. There's a few options.
Have a great time! When are you going?
-Alison