You need to know the websites that he’s clicking on.
If your child is very young, he won’t be surfing from one place to the next so quickly. He’ll need your help. But when your kids get older, they should know that the Internet is public and that wherever they go online, they’re leaving a trail that you can follow -- so he shouldn’t go anywhere that he knows is not right. If your child is being secretive, ASK her where she goes online. This should be an ongoing dialogue. You don’t let them watch any TV program they want, so why would you let them run wild on the Internet without your input?

Stay involved and participate!
Not just for safety, but for a shared experience. There is so much to discover online: music, science, travel, games, art -- the list goes on and on. The wealth of information and experience available could have a profoundly positive influence on you and your kids.

A Technical Solution: Buy Parental Control Software.
Don’t get an Internet filter with a feature of parental control. These tools tend to be less flexible, less intuitive, and frankly, they get in the way more than they help. The program I recommend is SafeEyes for both Mac and PC.