Yes, thats a good shout Cubed, also been there with our little boy - twice! London Zoo as well of course is an obvious choice if not too cold. Although our DS1 freaked at a snake that lunged at the glass front while he was stood looking! Both places expensive of course. The Museum of Childhood on the attached list is also very good - I took 3 u10s there at half term and they loved it.

Took my 14 month old to the Sea Life aquarium just next to the London Eye. At this time of year it was really quiet so she could happily toddle (and crawl) about. She loved it! It was nice and clean and warm too.
It's quite expensive (about £19 per adult) but if you travel from Clapham Junction get one of the 2 for 1 Days Out vouchers (you'll need to show a valid train ticket at the aquarium).