Problem in communication
I can't believe that these offenders strike it lucky first time nor can they have a one to one success rate. Obviously they must get rejections while they are honing their skills unless there is some infallible how to guide for the would be predator which I seriously doubt. Hence, for every girl these people snare there must be tens if not hundreds that don't take the bait. Which is where the problem in communication is, the problem is between children and parents. It's all very well telling your children of the dangers of the internet but they also need to know that should something inappropriate be suggested to them then they should be confident enough and comfortable enough to tell you.
Whether you then have confidence in the police to actually do something about it and follow up on it is a different matter.