My kids, begot by a history-crazy father, have already visited a large number of castles despite their early teenage years. Needless to say, they have considered every one of those castles an inexpressibly boring heap of old stones.
Remarkably, both these teenagers declared Leeds castle the best they had ever visited, possibly because we never got around to actually entering it. (Having been there before, however, I know it is rather sumptuous inside as well.)
What we did get around to doing : Saw the falconry show (love falconry!), got lost and muddy in the maze, watched a somewhat over-advertised large-screen video on the battle of Britain, walked in the park and looked in on the Dog Collar museum.
With a little more time, we would also have taken the punting tour around the moat ("puntagenet"!) and done the treetop adventure course.
For any history buffs among you, I will point out that the castle in question was built in the 12th century and subsequently taken over by British royals. Henry VIII lived here with wife no 1, Catherine of Aragon. And though it no longer looks quite (or at all) like it did in the 16th century, it remains a very beautiful castle.