“joking” kids can take that to heart if they hear that all the time and might begin to believe it. It’s emotional abuse. All abuse isn’t physical. Calling kids names isn’t the same as a dad saying he’s gonna kick your butt, because you know he isn’t actually gonna do it. But a kid being called stupid is a descriptive label that can actually harm their emotional well being. Tell the kid his behavior isn’t right, his actions are rude or whatever-tell them the way they are acting is “stupid”, not that they are.