How horrible! Do they realize that they're doing it? What would they say if you confronted them....
Say something like... the kids can't come over when Little Johnny is there because it hurts their feelings so much when you treat Little Johnny so much better than you do them. So rather than see their feelings hurt so badly, I prefer to keep them away.
I don't have children, but with my nieces and nephews I always try to keep things as equal as possible. I do tease them a lot and I'm always telling them 'Don't tell the others, but you know you're my favorite!' Now that they're older they just grin, roll their eyes and finish the sentence for me.
