There are so many things to do you won't get a chance to do all of them so just do what you can and don't try to see too much or you will get worn out. The museums are fabulous, especially the Natural History one (be aware that the Imperial War Museum is closed until some time in July). The main tourist spots are (obviously) really busy most of the time, for example shopping on Oxford Street but there are some neat little pubs and eateries tucked away down the side streets and these are often much quieter. Take care though and don't travel too far from the beaten path! Hyde Park is a nice open space and Speakers Corner is home to some colourful characters.
The Royal Observatory in Greenwich (pronounced Grenidge, not green witch) is different, you can stand astride the Prime Meridian which is the line that officially separates East and West, that might appeal to the youngsters? There are other attractions at the same place like the Cutty Sark and the National Maritime Museum all in an amazing location and you could easily spend a day there. Of course the kids may prefer to go on the Harry potter studio tour -
Warner Bros. Studio Tour London
Just remember that London is not really representative of the UK, it is very cosmopolitan and almost a world of it's own like many large cities. You need to get out of London to see the real UK like rural Gloucestershire where everything is slower paced and far more civilised! Anyway, enjoy, hope it doesn't rain (too much!).
