Kids love this place. It's right in the heart of Covent Garden so it's a good compromise - kids go to the museum, and then you get to do a bit of shopping. Loads of old buses on which kids can run around. Get a tour of the history of transport in London which is really fascinating. If you are into design there are often exhibitions of transport maps, posters, etc. It's open late on Fridays until 10pm. Best of all kids under 16 get in for FREE!