We use Spoonfed (
http://www.spoonfed.co.uk) which is a London events listing but it tags all the family events. I'm not sure if this link will work, but you can click here:
http://www.spoonfed.co.uk/simpleEven...nre=232&page=0
for all upcoming family events in London at museums, galleries, etc. So far it's the only event listing site I've found that does has family things! My kids (7 and 4) love the museums with interactive exhibits so that's always a good choice. Good luck with your trip!