3yo humping

My 3yo likes to lay on her bed or the couch and go to town. It's all outside the pants (for now) and the last thing we want to do is shame her in any way. We tell her it's okay to do it but it's private so she should do it when she's by herself. But I've found out she will sometimes do it at preschool during nap time which I don't want her to do. I've told her over and over. Is this even an issue? I know kids do this and it's all part of exploring their bodies but how do I know when it's become a problem and what do I do about it? via /r/Parenting http://ift.tt/2f3JE6u