It's nearly impossible to police the internet.
Generally speaking the only people the government attempts to keep away from the Internet and computers in general are major hackers who have been caught.
Some of them aren't suppose to touch a laptop or desktop. However I'm uncertain if they are allowed to have a "smart phone".
There may be a way to prevent pedophiles from viewing child pornography just as technically that's illegal for anyone in the U.S. This most likely would be a tracking devise of some kind.
Nevertheless there are porn sites who specialize in using "models/actors" who are over the age of 18 but could pass for age 14 or 15. Legally they're not breaking the law but the intent is still there to sell the "fantasy" of them being with an underage youth.
Having said that there is no way to keep them from seeing TV commercials that feature families, children or babies. The advertising circulars one gets in newspapers from J.C. Penney, Sears, Target, Walmart and so forth are everywhere.
The car next to you at the stoplight may have children in it.
If someone is so "twisted" that the very sight of children "turns them on" I don't believe there is a way keep them from ever seeing a child in person or via some form of media.