As has been mentioned many times by at least unity100 and BigDish in terms of effort but in the vein of houstonbofh: a system can be made to do this in a variety of ways. And maybe for younger kids some of the censorship is ok and off-the-shelf routers provide this easily. But the reality is, be a parent. If they are grounded and that is part of the grounding then take the power cords if they've proven they will use it when grounded. It is your roof they are living under, if they don't like it then suggest they find more amenable living conditions. Sorry, but I have 3 daughters the youngest 18 now and they will respect you more when you do that. I understand we want to use technology to ease our lives but raising children is not an area to do this in. At least not in this manner. If they circumvented anything I did I'd have to smile at that because could respect that. Would you rather have them discover something unseemly on the internet under your roof or would you rather have that first discovery be when they are on their own. Seems like a good learning experience with you present.

This would require work but could customized to your own desire. http://www.buglabs.net/products