Our two (Alex 4.5 yo & Sam 3.5 you) can quite happily spend a couple of hours on the Jay Jay/Cbeebies/ Kwaala & Boowaa/ NickJnr sites. Alex can turn on the laptop and navigate to the sites himself
We feel it's better than telly (but sometimes only just) they balance the tv/pc time with lots of outdoors play, colouring, learning to read etc.....but we do feel that they are looking at screens a bit too much.