Bully at playground

I love travel and I often bring my son for outing especially for fun namely going to playground and movies. I cannot believe but it is true there are bullies at playground, it started from one gal aged range eleven or more. I have not seen her around here before and she has started to calling my son names like fat boy or ugly boy. This is not the worst of it the other kids in the playground started follow what she says, so annoying! My son has no interest to play with them, even if he played with them it would be a while. They are playing catch but the catcher will need to close eyes counting to ten, they are not playing fair game so my son does not want to play. The gal is not local but she is good in Malay language and English. Most kids at the playground does not have people with them I mean like guardian. Some of them go there by themselves by bicycle, some with their maid. My son going to be standard one next year and next month only knows if he can get in the school. My sister in law was here yesterday asked me if I want another kid, I have got two boys they are enough. I told her having kids are costly and expensive, she agrees with me. She has one set of twins a gal and a boy.