I would sit down calmly with your daughter, and discuss that at her age she shouldn't be playing with other's kid's privates, and that the little boy shouldn't be showing his either. It may seem like a game, but it's definitely not.. Make sure she doesn't feel bad about it, but encourage her to stop the behaviour though in a calm and firm way. Hope that helps.
It is important not to freak out, as she might not tell you if there is a next time. Have a calm, low key conversation with her where you make it very clear that she is never to touch someone else's privates nor is anyone ever to touch hers, except for a doctor when you are with her. Tell her that if anyone wants to show her his or her privates, she should strongly refuse and tell a trusted adult, like her teacher or you. Compliment her for telling you this and let her know she should always come tell you these kinds of things.
Then, call the head of your daughter's school and tell them exactly what happened, including the names of the boys involved. You need to be convinced that the boys' parents will be told. The school needs to step up and not try to ignore this or dismiss it as kids just being kids. This kind of behavior can start innocent, but can quickly proceed to abuse. As it is, it was just inappropriate, but still needs to be corrected so the kids don't think it was okay.
I personally would not use the word "dirty." That association can lead her to feel that she herself is dirty. The word "private" is a better choice.
I dont think this is normal. Maybe your daughter thought it was a game. Try to tell her that it's not right and she shouldn't do it. say something like, "Sweetie, you're way too young to be doing things like this. If this happens again tell me and then i can help you. I know you might think it's a game but it's not and its dirty. Please don't do it again."
This is definitely not normal! You need to contact the school board and possibly the boys parents. Then tell your daughter that its not ladylike to do these things.