My child complained about being bullied at school. I feel like complaining to the school but I am good friends with those kid's mums and do not know how will they react or do I talk to them directly?

I would propose speaking to the teachers, get them to keep a look out for such instances and let them handle the bullying on the spot. This may avoid awkward tension between you and the other parents. Also, the teachers could intervene such that the children can learn that those are inappropriate behaviour. You can also try asking your child to report to the teacher when he/she is being bullied so that the issue could be addressed. It will be difficult for you to approach the parents without being present when the incident took place. Just some of my thoughts for your consideration. Regardless what you choose to do, remember to let your child know, so that he/she knows that you are attending to the matter.