YTA.

As someone who grew up being bullied for being muslim/brown skinned/ an immigrant, let's just clarify something here. Bullying isn't when someone states something that opposes your belief system. It's when someone in a position of power repeatedly uses that power over time to degrade you in various ways (physically, emotionally, socially etc.). Kids telling your kid, at lunch, that they don't think Christmas is real is not bullying. It might have hurt his feelings but that's life, there will be other people who say things that opposes your kids belief system.

I wish the kids at my school stopped at simply telling me my belief system was made up. They spent years verbally and physically assaulting me.

You are shocked that others with different cultures may say something about yours. Welcome to the real world. Your whiteness, and lack of problem solving skills is showing here.