When I was a Linux dad with young kids getting access to the net, I spent some time worrying about this. When I grew up, you might score a copy of Penthouse, but today, you might find "Two Girls One Cup," or back then it was probably goatse. Nothing I ever encountered in Penthouse could compare to goatse. What would stuff like that even do to the mind of a child? Not to mention all the pedophile stalkers and lions and tigers and bears. I felt I had to try to protect them.
In the end, I found some stuff some guys said I could use to create a net nanny thing. It involved reading several PhD dissertations, a lot of programming, and at least three rubber chickens. It was all way over my head, and I never got anywhere with it. The kids ended up having totally unrestricted, unfettered, free reign of the whole big ocean of sleaze and pedophile stalkers and everything, pretty much from kindergarten on.
They turned out fine, and eventually thanked me for not trying to net nanny them. As my now 21 year old son said once, "Dad, I never wanted to watch donkeys fucking midgets anyway, and you never had anything to worry about."
Both of them have switched to Windows, incidentally.