There is one tried and true content restricting system: parental supervision. While parents cannot monitor the children continually, all internet transactions are currently logged by ISPs. It is a relatively simple matter to make available to acount holders (parents), at the end of each month, a list of all websites visited including search queries. Concerned parents can cast an eye over this list, and click on any dubious-looking addresses or queries to determine its suitability first-hand.
While not foolproof, this would allow a dialogue between parents and teenagers about their usage, much like the phone bill does now for voice calls. This is a feasible, lawful and commonsense solution to the problem.