Assign each kid a day of the week. Say Eden gets computer time on Mondays, Jake gets computer time on Wednesdays and Johnny gets computer time on Fridays. You get all the days in between. If they argue and fight/moan about it, they lose their day altogether that week. Every other day they must use their imagination and do something else. We limit TV time as well. No TV before school, and only twice in the evenings (during the week) once they've had a decent amount of time outside/doing something else.
I noticed a HUGE difference in our house once I implemented this plan. The kids didn't like it at first, but they got used to it, and now they don't even ask for TV or computer because they are really enjoying themselves outdoors, and using their imaginations a whole lot more. There's no whining, grumbling or fighting that used to come with too much TV/computer time, and we're all much happier overall.