Parents, I've found a device that you will love and your children will hate. Just don't tell them you heard it from me, I have my geek cred to look out for. It's an evil video game timer called the GameDr.
It attaches to the power plug on your kid's console via a combination lock. Then you can program the amount of time left until it kills the power, losing your child's progress, and making him or her resent you. It will give them 10- then 1-minute warnings, so that's something.
Sure, they'll get their homework done, but is the rage they'll feel toward you for all time really worth it? We don't think so. But for $30, you're free to disagree.