It is a psychological problem. After all, when we are little, playing with someone of a different race does not matter. As long as the children are having fun (and fun is key), the little kids do not care about such things. It's only when the parents display their emotions (repeatedly) about race in front of their children, do their children start acting out what they see their parents doing. After all, little children want to be just like their parents (or any other person the child may look up to).