I wouldn't worry bout approaching her with it, but about finding the source: Who touched her out of curiosity? Or did she see such touching? Heard about it? I would be concerned rather than teaching her that she is doing something wrong (killer of self-esteem.)

All kids are curious. Don't panic, stay real calm. Ask your child very casually, why they touched this other child there. Explain that it is normal to be curious, but it may upset the other child, so it is best not to do it. Then suggest a game that they could play together, (as a way of distraction). Or you could try ignoring it first time round. Your child may have seen someone else do it, or may have had it done to them. You need to stay calm to get as much information as possible. Best of luck Sarahxx.

Teach her that our bodies are ours only...No one should touch her and she doesnt need to touch others...(good touch bad touch)

Its not that serious but at 6 yrs. the First grade teachers may get upset, so your child has to be told.
even if its as simple as we keep our hands to ourself.