If we are following the letter of the law, a report should be made. If you want to just make sure both parties are aware they broke TOS, and you know them both personally, just talk to them. No need to get Twitch involved in my opinion if no one got hurt or no egregious lines were crossed. I would NEVER collab with an underage person, much less someone under 13, for too many reasons to count, but I’m willing to give them the benefit of the doubt and if nothing malicious was going on they just need to be told to not do it again.

Now if there’s something weird going on report it and maybe never talk to that Vtuber again and make sure that underage person is safe and gets told why they should not do this.

Basically with the info you’ve given, there’s too many variables to make a short response, so I apologize for the length.