That is when the parents need to step in and give the little punk a spanking. While he/she is weeping and wailing, they need to be asked "How does it feel? How much this is hurting you is how much it hurt that other little kid. Do you LIKE how this feels? Maybe you'll remember this the next time you think about beating up on another little kid." Then something needs to be taken away, a toy or a privilege, something which the perpetrator VALUES, not to be given back until he/she spontaneously demonstrates some sort of kindness or consideration for someone else, showing the lesson has been learned.