I can't recall, but I think your DS would be a good age for THATmuse, info
here. They do "treasure hunts" at several museums as well as street hunts- which is basically just finding clues or taking pictures, but it gets kids much more involved in museums and the details than just wandering around looking at "stuff."
We did
biscuit decorating in Notting Hill as well as a went to the market in Notting Hill.
We love London parks. Princess Diana memorial playground is next to Kensington Gardens (beautiful, easy photo opportunity).
London walks- free walking tours.
Found the London Eye to be a HUGE waste of money. Kids will want to do it but it's so expensive and just so not worth it. But near the base of the London Eye on the South Bank of the River Thames are some good American-ish, kid friendly street food and street performers. Easy walking distance to Big Ben (or more accurately as my kids learned and remind us from their London Walks tour- Elizabeth Tower).
Be sure to grab that iconic photo in the red phone booths scattered around London.