I would NOT discuss this with the child and say this is something you do at home. I didn't suggest that. I wouldn't discuss it at all. I would just block and go about my merry way. I don't think I would talk to the parent about it much either unless they brought it up. I would just let them know that I put a blanket sleeper or footed jammies or whatever block that fits for any child that puts their hands in their diaper area at nap. Doesn't matter to me why the kid is doing it.
I adore simple. I live for simple. I would find a simple solution and go about the business of taking care of the group.