This is yet another case of modern parental paranoia. Your child's greatest risk factor comes from people already known to you - statistically Uncle Bob is many times more dangerous than a random stranger.

As a society we have a tendency to focus our attention away from where the data leads us. We will act extremely suboptimally to prevent an incredibly remote risk, while ignoring much more likely ones altogether.

If the goal is to prevent child predators, we would do a whole lot better by starting with the people already in our immediate lives, rather than worry about a stranger on a bus.