Pretty trusting with your kids to someone you don't know. I like the idea of the background check etc. Even so, I think it would be very hard to leave your kids, alone, with someone you don't know, have no idea about anything about them, and, never even met them before. Westerfield probably seemed normal too, without a history which would show up on the computer, etc. Just my thoughts.
Now, with the group activities, where there are a number of kids, in a public place, sanctioned, and run, by Disney, held out in the open, I don't feel nearly as apprehensive about leaving the kids.