realistically there's very little you can do. You can block at the dns level or content filtering level, but any reasonable kid will get around such a measure.
You can enforce operating system restrictions, but then youll have to avoid them booting off a usb pen into their own O/S.
You can block at the network level, but there's nothing to stop them using wifi to steal your neighbours connections and downloading whatever they want.
You can even do do all of the above, and then they'll use their iphone to go to the sites you wanted banned.
Point is, you can try all sorts of things but the likely outcome will simply be resentment and causing them to look for ways to break the measures (with success in all likelyhood).
The best measure you can take is education, teach them whats good and whats bad, and why. Teach them how sites can infect them, how viruses work, and why computer laws exist.
Do this, and you may even steer them into a career.
edit: Oh, and i'd say theres FAR more harm done by using a computer 15 hours a day sat on your butt every day than viewing adult material. Being smart will prevent their brains being destroyed but a destroyed body is a lost cause.
My summary: Limit the time on the machine, not the usage patterns.