I think a lot of it is two things:

1. Hearing it from home, obviously. Some parents will say things at home that they won't say in public, and kids don't learn that unspoken lesson of not saying those things. It's the classic Northern-style racism.

2. A lot of it is from online. You see a lot of casual racism online, and you also see a lot of POC online making the same jokes about themselves, and I'm not sure kids understand the whole "It's ok when they say it about themselves as a joke, it's not ok when you say it" thing.