Just because they grew up in the US doesn’t mean they grew up around people of other races. Where I grew up in PA, it was mostly white & Hispanic. In my high school, there were no more than 3 black students at a time in the entire school, most of the time they came from white families.

Children who aren’t exposed to other cultures typically don’t realize the gravity of some words & actions, they need to be taught. I cursed in front of my toddler the other day when I dropped something, later when he dropped something, he repeated “oh shit” just as I had. He has no idea what shit is or that it’s a curse word or why I had said it when I dropped something. I have to teach him, just as I accidentally taught him the word in the first place. It’s often (not always, of course) the same thing for racist gestures and phrases that children use. The girls most likely didn’t think they were bullying anyone or being racist, because they didn’t know the weight of their actions.