I'm on the Advisory Council for a product from Symantec/Norton called OnlineFamily.Norton (www.onlinefamily.norton.com).
It really is very good. It's free for the rest of this year, too. What I like most (as I have 4 kids) is that it's flexible. We can set appropriate rules and limits (can even track/limit how much time each child gets on computer) for each child.
Definitely worth a good look.