there's so much to see, there's a temptation to try and do too much, which you should resist. (we took a visitor from Estonia on a daytrip to London a few years ago - she made a list of what she wanted to see/do, and we did it all. the poor girl was completely overloaded at the end....)
decide what you want to do - culture/history, the arts, fun - if the latter you could do worse than base yourself on the south bank. a spit from Big Ben, the London Eye, Sealife, London Dungeon - and your young'uns will be fascinated by the street theatre artists who usually perform by the waterfront. plus there's lots of eating choice and is well served by public transport.