For families you can't beet British Transport Museum in Covent Garden - it's free
for kids and each adult ticket is valid for as many return visits as you like for a whole year!!
Loads for children to do - a trail to follow and enough vehicles to sit on + plenty of interesting stuff for the grown ups too!
Good cafe and they're happy for you to sit with your own packed lunches.
Also every saturday from now until Christmas there are reindeer in the piaza for children to pet!
We usually then walk down Whitehall and look at horesguards parade then try to time it right for Big Ben!