Mild discouragement

...or maybe you've even experienced or heard of something similar?

Anyone who spends a significant amount of time with a 5 year old sees them do stuff like this.

You should mildly discourage her from doing this. She doesn't understand how her actions look to other people, and it could be a tramatic experience when she realizes it, making her feel embarrassed and ashamed. She probably won't make that leap at age 5, but the older kids might remember it and tease her about it later.

I assume you aren't the responsible adult here if you went to the bathroom to ... err ... take a cold shower. But you can do her a big favor by pointing her in the right direction. Don't be stern or insist that she not hump the sheets, just imply that that's not part of wrestling, or that it's not lady like, or something along those lines. If she ignores you and keeps doing it any way, show her less attention while she's doing it, and more attention when she is doing something else.

It's okay to fantasize about it later, but when she does this in your presence, you need to mildly discourage it to help her become sexually healthy and happy later on.