Keep letting them play.
Realistically kids are safer from the threat of Paedophiles nowadays than they have ever been. Go back to when you were a kid, and there was exactly the same amount of nonces per person as there are now. Differences were that if a child was attacked a lot of the time it was brushed under the carpet as an embarrassment, kids didn't know what to look out for, and the predators were hardly ever caught.
Fast forward to the present, stranger danger is taught a lot more strongly, sexual abuse is taken seriously, parents know what to look out for, DNA testing and the like means most nonces will be caught.
I understand the urge to protect them, but if you do it to much they will never grow and learn. Kids need to learn that falling out of a tree hurts, so the next time they climb they will be more careful.
If you want to be more comfortable about the situation, then ensure they know about stranger danger, and speak to your neighbours about a Neighbour Hood Watch-type setup