First off, I know the topic needs to be discussed, but it just breaks my heart that it does.
I like GeoMamma's approach. I also think it's important to frame it in the context of "most people are good, a very very few people might be bad".
Also, an attempt at promoting peace of mind: I think as adults we need to be reminded sometimes just what sort of chances we're talking about. In 2007, out of 74 million children in the US, 518 were abducted by a stranger. That's one in 143,000. You're ten times more likely to be struck by lightning or drown in your own bathtub than to be kidnapped by someone you don't know.
