I have dealt with this issue with quite a few of my friends and colleagues. No matter what you do or say in a parenting manner, these kids are going to get on the net and look at what they want. Without a "tool" to enforce the rules, you are pretty much dead in the water. Kids these days are so tech savvy that you have to use a sledge hammer where a ball-peen would have worked a few years ago. You use a program on the computer and they find a way to uninstall it or bypass it. You try a DNS service and they use Tor to bypass it. The only thing that I have found that works "MOST" of the time is a security gateway like Astaro or Untangle. And then you have to lock the modem and gateway in a closet so they can't get to it. These are both free for home use and work very well.