My child's Apple Account affects my contacts on iPhone

my sons phone (he is 10) kinda thinks it’s mine. He’s got his own Apple-id. Setup as a child. It changed some of my contacts names, as it was written in his contact list. And other wired things.. we have a family calendar and now everything is written duble in my phone (when I tried to add him) I tried removing him, but it’s still like this. Have anyone else experienced this? And found a fix?

[Re-Titled by Moderator]