Feeling a bit cocky today so will offer Mighty Mom's smartmouth retort. If another child is bullying your child, invoke the Aggression Rule of Reciprocity (ARR). While the Christian solution might be to tell your child to turn the other cheek and forgive, practical experience has taught us that this will do nothing more than incite the bully to further bullying.
Here's how you nip that kind of bullying behavior right in the bud.
You march your little Petey Picked-on over to Billy Bullier's house.
You have both kids sit down and observe carefully.
You then proceed to mimic exactly what BB did to PP, with you in the role of BB and BB's adult guardian in the role of PP. It works best mother to mother (or if there is no mother, a father, grandparent or foster parent will suffice).
This way, Billy Bullier gets to see how ridiculous his bullying looks.
Your child gets to see what a hero you are (yes, even Moms can be heroes). And Mrs. Bullier gets to experience the consequences of raising an overly aggressive child. As for you, you get the satisfaction of teaching everyone a lesson.
Bonus points. Hopefully BB's bullying involves some form of physical violence. I always find it extra rewarding when I get to punch a bully's mother in the nose:-)!!!