I feel for you. In our house computer games, like Minecraft, began to consume all our children's (12 and under) time and attention --- they wanted to do absolutely nothing else. So, we made a rule, no computer games, except on what my wife and I called "Technology Tuesdays." At first the going was really tough, but it seems to have worked. The children now enjoying playing computer games on Tuesday (which turns out to be only a few hours after school on Tuesday), but are back to pursuing other interests the rest of the time.

On the other hand, for truly educational content, like Khan Academy, they have unlimited access.