[nikkianni]

I don't think I would panic yet, but I would supervise their play. And I'd do it from a distance. Sort of like you accidentally did in this case.
Because kids just don't act like themselves when adults are around. And ultimately if there is a problem, you not only want to protect your child, but try to help the other two boys as well.
I think the good touch bad touch conversation may be a good idea too.