Children are bombarded with images of whiteness
as the default. Teachers, commercials, cartoons,
etc etc, you name it.

You sound naive.

Children don't live in an Alien Supremacist society,
so dressing up as an alien isn't going to impact them
the same way as dressing up as a white person.

Sure, he's "just being a kid", but what that means for
a Black child in a white world is totally different
from what it means for a white child.