Thanks for all the replies. I'm starting to compile a list of 'must-dos" and then some nice to dos if possible. Will probably avoid most of the Harry Potter attractions as my kids have passed that stage in life. Now I've got to figure out the difference in a National Rail pass and an oyster card. Any more hotel recs?
I've stayed at the Park Plaza County Hall, and the Park Plaza Sherlock Holmes.
The County Hall is really well located, just a short walk away from the London Eye, and across the bridge from the Houses of Parliament & Big Ben. It's also easy walking distance from Waterloo Station. Nice, clean, modern rooms, relatively reasonably priced for London.
The Sherlock Holmes is on Baker Street (surprise, surprise!) in Marylebone. It's also well located, because it's easy walking distance to the Baker Street Tube Station, which connects to multiple lines. It's also easy walking distance to Madame Tussauds' Wax Museum, and Selfridge's Department Store. It has a Pret a Manger right next door as well as a Tesco's (drug store/convenience store). LOADS of character, but because it's made up of 3 buildings that were connected as one hotel, the rooms are all configured VERY differently, so you'd need to make sure you got one of the rooms configured to handle a family. Also not horridly expensive for London.
The Oyster Card is mostly for local stuff around London such as the Tube, the bus, some local trains and the Thames Clipper, which is a water ferry that goes up & down the Thames. The Thames Clipper is a nice way to get from place to place on the Thames, like from the Big Ben area to St. Paul's Cathedral. Plus, you get a really nice view of London from the boat, and you can sit for a bit.

The Oyster Card is reloadable at any Tube station, and you can get a refund of whatever is left on it when you leave. There's also a daily cap on how much you spend on the card, after which, you aren't charged any more. I'm not sure what that cap is.
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