I would address issue with the school personel : teacher, principal, counselor
I would also address the issue with the childs parents.
If a child were to hit or physically harm another child, the school or the parent would be held liable for the damages. I think it should be handled the same when the damage is emotional.
I would not directly confront the child. I am not sure about leagally, but any time you address someone elses child wthout permission to do so there is a great risk of issues with the parent - regardless of whether you are right or wrong in your statements.