Would you tell the parents?

[Sandbox Sally]

I haven't read the replies, but in this case, I would not tell the parents. I'd clean his undies and put them back on him. It sounds as if he's scared to let his parents know (I can speculate as to the reason, but I won't). I would make this a trust bond moment for you and this little guy.

If he'd reacted differently, like maybe if he was super embarrassed in front of you, I might mention it to the parents, just because he was upset, but since he seems to find it important that parents don't know, I'd give it a pass.