What helped in some cases for me:
- There was a boy bullying our daughter on the playground so she didn't want to go, he was 5 and she 4 or so. One time I went there when his mother also was there and scolded him really loudly infront of everyone. The mom came running and said she will get her husband who does karate or whatever, I just told her to do it but if her son ever talkes to my daughter then I will deal with him if she can't.
- A girl was biting our son in Kindergarten so he would come home with bite marks, she was half a year smaller than him, he was about 3. We told the principal that we expect them to move this girl to at least a different group so she stops biting him, better that they remove her from that Kindergarten. It went a bit back and forth but then her parents moved her to a different Kindergarden.