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 speak to the mums direct if you already know them. Just be aware that most mums do not like the word bully and may deny their kid bullying yours. What you want to do is call and say "hi this is .....I need your help with something..."