Museums are all over London and cater to both the broadest and the most focused of tastes.
The king of them all is the British Museum, the subject of its own web site and many guide books. Guidebooks will direct you to the high points, such as the mummies, the Rosetta Stone and other treasures; the web site includes a calendar of activities for kids at the Museum.
We'll share an idea and one strategy with you. The idea is to build up some anticipation, age appropriate, in your kids. So, we read “Alice and her Adventures in Wonderland” before seeing the Lewis Carroll exhibition, and saw “Prince of Egypt” on video and then went to gaze at the mummies.
Now for the strategy: the museum is huge, full of stairs and for some kids brought up on Scooby Doo, dull, even the mummies, who don't talk or chase you or anything. So, my wife and I chose a few things the kids would likely enjoy (one child was surprisingly fascinated by the displays of ancient jewelry) and mapped out a path that would take us past things we wanted to see on the way to the things the kids would like. It wasn't ideal (ideal being defined here as Mary Poppins appearing in the museum, taking our kids off to teach them to speak in a British accent, while my wife and I toured), but we did get a look at many things we know will be there again when the kids are a bit older.
- The Science Museum (National Museum of Science and Industry, in Kensington) has a basement area with hands-on stuff for younger kids (“yes, you can touch it!”).
Can't get any better than this: the big London Museums are now free all the time, so stop your Smithsonsian envy.