How far are you willing to take it? Given that most child abuse happens at home, and most abusers are family, it would seem that mandatory surveillance cameras in every residence would help quite a lot; much more so than anything you could do online, anyway.

Of course, this is also very intrusive, but if your threshold is "it's okay unless it's worse than the child abuse that it is likely to prevent", then it would seem to pass that. Unless you have other criteria?