Then you've become a responsible parent havent you? You've installed the proper functions to prevent them from access and that is your right.
Government starts at home and not in Washington DC.
Not prevent, monitor. The only way to prevent is to completely lock them off from the Internet, but then they wouldn't have learned their bounds for when they're outside of my control.
Quick and easy example: Parent installs blocking software, kid learns to clear his browser history/cache/cookies, he has a friend set up a proxy and creates an encrypted tunnel there to get his 'net porn from home. It's easy, and the software is available free online. You'll only know if you're actually watching your kid or notice all that strange port 22 traffic on the router.