I can understand where you are coming from. But I have read lots of post on here from parents who's children do exploring and they think it is associated with sexual abuse. I did it when I was little. I explored myself and my friends. It was nothing sexual because all we knew at the time was it felt good. Like scratching an itch. What you might watch out for is the curiousity getting out of hand if it was just curiousity to begin with. Which I think it was. I take it this was the firs time. Just tell her right from wrong and her private parts are her private parts and no one should see or touch her there.
I hope this has helped.
Tell her what the mother said and ask pertinent questions , it is possible that she has learned this behavior and it would be good to check that out, ther is also the possibilty that the other child started it first and your daughter copied, did the other Mom tell you what her they said when caught? I read your other post this doesnt seem like a rude post to me, you are a concerned Mom asking for others opinions. Fist thing sit her quietly down tell her what the other Mom told you and see what she says.