Two things seem to be occurring. In a general sense, if all your daughter was experiencing is the pleasurable sensation of touching her genital area, this would be regarded as entirely
normalNormal saline flush and all you would need to do is educate her about doing this in a private space, such as her room, not in public areas or in the presence of other people.
Her particular comments, though, would not be expected. She has learned those from someone, and it is sensible on your part to voice some curiosity about this. Definitely mention it again, perhaps with both you and your wife present, in a very calm quiet, supportive way. Also, speak with her teacher about this and see what she might be able to tell you. If you know the parents of any of the other children in her program, ask them also. They may have some information that you might find helpful.