So....you let a complete stranger take your 7 year-old niece to the fair

Things, even involving kids, were a lot looser back then; however, I found it implausible that, even in 1962, a guy would hand a kid over to some dudes he just picked up hitch hiking.

Implausible. My guess is that the uncle might not have had the best judgment? The fact that he would pick up 2 hitchhikers with the little girl with him is questionable. And then he disappears after an accident 20 miles away, so he went driving long distance without the girl.

So the uncle was a bad guardian?