How old are the children? 4 year old or younger preschoolers?
My son for the last two years has had an aggressive child follow him and try to harm him after school. I believe the other child,who is older than mine, has some behavioral issues anyway. In order to solve the problem I had to tell my son he's not to fight or wrestle with anyone, tell the other children directly while making eye contact that my child is not allowed to fight, and bring it up with all the other parents.
These things only helped sometimes and for short period of times. Finally my son came home with bruises on his neck and I told him to punch anyone who touches him in the face. Sure enough he punched the other child and told him "my mom said to do that". So not way I originally wanted it handled, but it did fix the issue for the biggest part.