I feel like that's normal for them to be curious because boys and girls are different. I remember doing the same thing with a boy friend when I was about 3.
I told her mom about it, and she got really upset, saying that she wasn't comfortable with it, saying it was unacceptable and not normal. I talked to the kids about it, explained that boys and girls have different parts, but they shouldn't be showing their private parts to other people.
Does anyone NOT feel like it's normal? Should we actually BE concerned? I feel like as long as we acknowledge their curiosity and explain things to them, it should be left at that.