This should not be happening and I’m sorry your child had to go through this and I understand your concern as a parent. It’s scary and should be taken seriously by Blizzard.
You don’t have to let your child stop playing the game they can just leave all forms of communication in-game which includes not chat from enemy team or own team and no voice communication. This will guarantee 0 communication from anyone else in the game. It’s not your child’s fault this is happening they don’t need to stop having fun because of some idiots on the internet.
edit
I am not a lawyer but your child is a minor there might be legal precedent here. You should contact Blizzard directly