What is there to explain? The children isn't going to see a black maid and think "Oh, so that's the only role for black people - servants! Fetch me my formula, nigger"
Children see a maid that is black in a cartoon they like, and that's as far as this goes. They have no idea that it was degrading at the time of production, nor do they give two shits about what people 60 years ago use to laugh about. Children don't apply what they watch to the context in which it was made, that argument doesn't hold water. If a parent feels uncomfortable with that, tough shit, you're going to have to let your kids watch stuff without projecting your own prejudice onto them.
Edit: Scratch that, they don't have to, because they're managing to get these things banned instead. Let's keep the existence of skin color hidden from our children as long as we can. Hell, let's segregate schools and communities, that way they don't ever have to see a person of different color at all