See, I'm torn on this one. On the one hand, yes, you have an 11 year old boy who could be demonstrating behaviour that might be consistent with someone who is being abused.
On the other hand, the accounts are somewhat vague and inconsistent (e.g. the girls showed him theirs first; what really was the age difference).
Is there any chance that this poor kid is the unfortunate victim of reading too much into the situation?
I think a lot of kids play these sort of games in pre and early adolescence... Spin the bottle, truth or dare, catch and kiss. It only reflects a natural curiosity.
How can you tell the difference if you don't talk to the child's parents or gather a bit more information? If this boy were your son, would you prefer that someone pointed out to you what was going on, or that they called the police straight off?
I remember playing some of those games when I was that age. And trust me, we were so innocent... The worst dare we could imagine was a kiss behind my friend's horses' stables without witnesses...
Now, maybe things have changed, but judging from the kids I know, I am not sure they have changed so much.
I just worry that there is another side to this that might be about a curious 11 year old little boy and not a dangerous sex offender.