Watching this thread with interest as a friend is bringing her kids to visit in the summer. I was thinking about the Kensington museums as suggested up thread. I also thought we would do a day on the south bank starting with a photo opportunity at Big Ben/Houses of Parliament -> London Eye -> Wander past the street market, Tate modern, Globe Theater, Golden Hinde, Tower Bridge -> Tower of London (if time). Lots of opportunity to stop off for food or have a closer look at things if they want to. If the weather is nice you can just amble around watching the street entertainers and trying different foods from the market.