The only real way to do these things is with proper parenting and supervision - establish a culture in your house that the PC is something they only use with your explicit permission and when they're allowed to. If that's the case and they're ever-so-slightly scared of not doing what they're told, then a software solution is much less important.
The reason I say this is mostly that software can never do what a parent can do - even if it blocks something, it can't let your child know how upset and disappointed you are that they've broken your perfectly-reasonable rules.
More to the point, any software solution can be circumvented if someone wants to do it enough.