Cadwiz

I just had lunch at my grandmother's house. While I am there, she tells me the story of what happened earlier today.
A man offers to give a young girl (8-9?) a ride to where she was going. The girl gets freaked out and runs up to grandma's neighbor to get away from this guy. The neighbor asks the guy if there is anything he can help him with, the guy speeds off. Neighbor calls the police and gives description of guy and license plate number. Police pull guy over not too far away, run check, find out he is a pedofile from 1985. As it turns out, it is not illegal for him to offer a young girl a ride. The police regret to inform them there is nothing they can do.
I realize the police have to enforce the law, and I am not downing them in any way, but what is protecting the kids from people like this? Is it that they might get in real big trouble if they get caught with a dead kid in the trunk and can't get a good lawyer? Our system really needs to be fixed. My thought was 6-8 months of legal vigilante justice. What are your thoughts?