The Imperial War Museum London is worth visiting. They have a lot of hands on items that will appeal to children. There are displays about spies, what life was like in WWI and WWII.
It has a great exhibition about the Holocaust that I have not seen anywhere else. It really goes into depth and help people understand what people went through.
The best thing is the museum is free as are most museums except the exhibitions. You need to take the tube to Elephant and Castle and then its a short walk.
Really worth the visit and children friendly.
American to Britain