Lambchops - heartily recommend Cadogan's 'Take the kids:London' book -loads of hints and tips of things to do, especially loads of free stuff (not sure what your budget is).
We had a London trip this summer with dss 4 and 6, and particularly enjoyed London Eye, National Gallery, Tate Modern, Science Museum, Natural History Museum, Cruise on the river, walking through the parks, Going to Kings Cross to see Platform 9 3/4, the British Library, the British Museum, London Transport Museum ... Open Top bus tour was good way to "see the sights". Disappointments were: Waterloo - you couldn't actually see the Eurostar trains! and Buckingham Palace - the boys just weren't particularly impressed. (We didn't go in or anything).
Depends what you're into really. We wanted a bit of 'culture'. In the National Gallery if you go in the left hand side (Sainsburys wing), and ask for the micro gallery, you can browse the piccies on computer, choose half a dozen or so, and it will print out a map for you locating the chosen pictures - fantastic way to visit a gallery, especially with children.
Science Museum has great website, and my kids coule happily spend a week there. If you're going in half term try and get there for 10 am when the doors open as it will be quieter. Head straight for the new wing or the basement, then do a special interest gallery later. On your way in go to the bookshop and buy a children's trail for a couple of galleries - £1 each, they are great.
I'm happy to give you more details of stuff you're interested in Lambchops, but the choice is pretty limitless, so give us some good ideas/areas.
When my kids are a bit older (like yours) I would definitely book tickets for a big West End show.
How long have you got there?