I would just walk up to my kid and say....now Sebastian*, you know that wasn't very nice...now draw a picture of you two playing nice and then we're going to his house and you can give him your I'm sorry card.
Why you little $#!+ -- why are you drawing razor blades in your picture....?
Okay, time for plan B... Elroy?
* - Does not have a child named Sebastian...don't worry.