i would treat it like you would if it were your skin kid. if they were teasing your skin kid and teaching him/her to be a bully, you would certainly put your foot down. This is no different. tell them to either stop or leave her alone. if they don't understand, then shame on them. i wouldn't worry about if they get upset or not, because they clearly don't worry about if they upset you. so just go with your gut and tell them how it needs to be.
good luck
