My brother once broke his hand when he was trying to punch me to get me off the computer and he missed and hit the chair, that'll teach him.

However, I sort of think such rigid things as a computer programme aren't really the answer either, just a bit of good parenting, instilling good practices in your children etc. etc.

As far as the programme you've written is concerned, I believe there are many already, all sorts of things for limiting time of certain users etc.