I think it's a bit more than the number of children, although that's strongly correlated: I think it's about the likelihood of children dying from other causes.

I think it's a bit more than the number of children, although that's strongly correlated: I think it's about the likelihood of children dying from other causes. If your children have a 30% chance of dying from disease / malnutrition / war / etc. then a 0.0001% chance of them being abducted by a stranger not only is irrelevant, but feels irrelevant. If all of your friends have lots children from commonplace causes, you're not likely to pay much attention to the story about the babysitter of the friend of a friend whose son disappeared while going fishing. On the other hand, if 99.9% of children live from one year to the next, incredibly rare events are now higher up on the list of dangers; if you don't know anyone personally who has lost a child, the tragic third-hand story about a missing child feels far more immediate.