London Eye has a playground next to it, for letting off steam while you wait in the queue. Walking to London Bridge takes about 30 minutes, but you can walk over to the north bank on the Millennium Bridge. Great views from Waterloo Bridge too.
There are loads of nice restaurants in London that do a set lunch menu of less than a tenner for 2 courses.
If you want to go to Kensington, there are the Natural History, Science and Victoria & Albert Museums, and a half hour walk will take you to Kensington Gardens and the Princess of Wales playground, and Kensington Palace.
I have been taking children sightseeing around London for a long time, can you tell?!
I see the Duck tour a lot - you can catch it from the Eye and it looks fab.
For how long will you be in London?