My son was touched on his private part. What should i do?

I have a 5yo son that was touched “inappropriately” by his female cousin who’s older than him by a year. It was after 2 days when he confided it to me that his cousin touched his penis and even referred it to a toy then touched his buttocks too. It happened in my own house when they were playing inside the room. I know it is natural for their age to be curious but I am bothered by my neice’s parents action towards this issue. I called the mom of my neice telling her what happened but she and her daughter denied such thing happened, saying that her daughter only touched his clothing. From there, I immediately felt that, to them it isn’t a big issue so I let it go and focused on the welfare of my son. After some time, my son was dreaming, talking in his sleep saying “she touched my penis then after that my buttocks” and that made me message the parents of my neice wherein they didn’t respond at all which lead me to block them and decided to call a doctor for a professional help for my son if this matter will affect him in the future. Also, for my sanity because I couldn’t work properly anymore and I am having a hard time processing my actions. As a mom, did i over react in this matter? What would you do if you were in my shoes? I need some enlightenment.

I don’t think you overreacted. Little kids his age have no reason to lie especially about something like that but at the same time I have seen similar situations like this and it did in fact turn out to be a little boy touching the girl because he was curious about her clothing. At that age they don’t understand what they’re doing or that it’s not appropriate. But still, he did not like this & now he is having troubles so I would’ve done the same no matter what the niece thought she was doing. Her parents need to talk to her about that. I know you blocked them but i would honestly send them a message saying it may be because she was curious, but even then those actions are no okay to do.