The best answer I've seen online for control kids internet usage, especially older kids ran something like the following:
Tell them that everything they have free access to the internet to research, play, and learn. However, What they look at will be tracked, and the logs reviewed. If they find something unsuitable, there will be an open frank discussion with both parents on why they feel it's necessary to view the material.
Nothing scares a teenager more than being faces with questions from Mommy on why they googled something perverted.
But, for younger kids, we just limit they times they have any access, using it as a reward after homework and chores are complete. The computer they use is in the living room and facing the room with someone around to monitor it. The younger kids tend just to play education or flash games.
For the older ones I've used the built in time restrictions that the mac has to limit usage, otherwise they can get a bit obsessed.
Also, you can put limits on the network level to stop the teens using phones, iPad, and kindles late into the night. On school nights it cuts off earlier than the weekends.
Also, below a certain age they have to give up their passwords so sites online (Facebook etc) can be monitored. No different to real life really. We know their friends and friends parents, no reason not to keep an overview on their online activity.
However, in everything they have a large amount of freedom - it's not about censorship so much as it's about keeping them on a good path and not wandering too much.