My kids are going to be needing something like this in the next few years, but things will be changing a lot between now and then I am sure. When this stuff was starting to become big, years back, I recall Net Nanny was one of the leaders in terms of popularity. Might want to check in to that one, as well.
As of right now, my kids aren't old enough to even know how to get in to trouble on these things. I have some games and a great program called Kidzui installed so that they can be entertained and learn how to use the computer fluently. This program also allows me to lock the interface so that they can not get out of it, although that can be easily subverted... just not by them... yet.