Go down town, near the London Eye. Even if you don't ride it, it has a carousel, human art, and... the kids love this, a "headless" person. I loved it there. And also, I'd skip the parliament section, it tends to bore kids!
Have a look at the merlin annual pass - great value even if you only visit half the attractions once - even better if you are planning a longer visit - http://www.annual-pass.com/ it also has links to the websites.
Check out the British Tourist Authority's "family ideas" website:
http://www.visitbritain.co.uk/things-to-see-and-do/things-to-do-for-people-like-me/family-ideas/index.aspx