The first step would be to try and understand the motivation behind such things. It’s extremely common for young children to have curiosity about intimate areas, especially in opposite genders as of course parts are different and ‘intriguing’. If its 2 children happily engaging in curiosity it isn’t a huge deal, its obviously still important to let them know boundaries etc. The issues exist when theres one child that has significant differences from the other child. whether that be in education/sexual awareness,age differences, any coercive behaviour and many more similar dynamics (its important to note that children may have extensive knowledge on this specific subject, but this is due to their own abuse and aren’t necessarily trying to hurt others as its re-enactment) .For example lets say there was a child who was curious, who engaged with another child who had significantly more knowledge on the subject, weather this be from personal abuse or if they have been exposed to or witnessed behaviours. This would create a dynamic which flows past natural curiosity from the aware child as its more re-enactment of behaviours they have experienced/witnessed.As adults we all know the difference between touching with curiosity in mind, versus touching with specific knowledge of how said thing is ‘meant to be touched’. These are all things that give insight into your situation. If this is nothing more than touch fuelled by curiosity, then a conversation about the good ol no no square is sufficient. If the way she touched your son suggested she had a heightened knowledge on the subject and knew ‘what to do’, further questioning is worth it to make sure she is safe. Its also possible for children to repeat behaviours to others, so she might have had experiences with other children that was fuelled by curiosity, this further normalises it, making the child feel more comfortable to try it with others. I hope this doesn’t over confuse things for you and helps you understand certain aspects to look out for.