Yeah, because we all know there's a huge hand that comes forth out of computer monitors and straps kids in to watch porn.
Apparently, you guys never heard of NetNanny, CyberSitter or other parental controls provided by the lovely free market and want to blame society's failures on strawmen.
Filtering software on a computer is like a mechanical safety on a gun -- good thing to have, but you'd be a fool to trust it completely. For younger kids, (I don't have any of my own), I would use a white-list system -- Each stie they want to visit, each person they want to IM, each person they want to receive e-mail from, would have to be approved by me. And as other FReepers have mentioned, keep the computer in the living room or somewhere you can keep an eye on things.
I'd probably let a kid have a computer in his room at the age of 16 or so -- but with monitoring software installed, like a silent alarm, to ping me to anything fishy. By that age, they're only a couple of years away from college, and you won't be able to control what they look at in their dorm rooms.