Saw my son get bullied at recess

Jo
What advice do you give your kids about bullying? 
I was parked in my car waiting to pick up my 7 yr old son after school today. The kids were still at recess, and I saw an older punk ass girl push my son(who's small) and then twist his wrist. He did nothing to provoke it..I had been watching him for ten min or so..
I wasn't bullied as a kid, I'm so sad for him😔😔😔😔😔😔😔
I'm worried this happens with other kids too.