Whatever monitoring system you're comfortable with for home computer use, don't forget that it doesn't cover what they're doing at Johnny's house where the rules may be less strict. Also, today you can access the internet from your XBox, cellphone, etc. And now we have "sexting"
abcnews.go.com/Technology/WorldNews/story
Short of extending the human gestation period to 18 years, there is no easy answer.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken