You have not mentioned what ages the children are, but hopefully the above two links will help.
I have always loved the parks - Regents, Richmond, Bushy - as well as the museums - Science, Natural History etc.
If they are a little older, then a city walk or visiting one of the bigger attractions such as the outside of Buckingham Palace or going to the Tower of London would be cool.
If you want to do something else, then you are not far from Legoland, Regents Park Zoo, the new Harry Potter theme park, London Aquarium etc.
Or jump on a train and go to Leeds Castle or Brighton for the day. over 6 years ago