Highly advise you consider doing at least one of these while you are in London:
http://www.walks.com/
Even my then eight year old daughter loved them and there's something there to interest everyone.
I think the bus is expensive, but it's a good way to orient yourself to what's there and get a general overall feel for where everything is. It may be a good plan to do that the first day, and not plan too much.
Three days should be plenty to see the highlights in the city. Definately don't miss the Tower of London!