We have constant wrangles about this with DS1(8) and DS2(6). Neither watches much t.v. but they both love computer stuff (games, Mathletics, programming in Scratch, cbeebies site, making Powerpoints about the Second World War). Some of the stuff they do is very educational (like the Powerpoints!) but I just don't think they should spend too much time sat still and looking at a screen.
What boundaries, if any, do you set around screen time? I don't really want to get into screen time as a reward for good behaviour - I just want some sort of yardstick for what's reasonable.