Nate knows that his dad has dark skin and his mom has light skin. He knows that his grandpa has red hair. He also knows that some dads can have red hair and not all grandpas do. He can tell the difference between a man and a woman. But, if a dark skinned and light skinned person are together, he still thinks the dark one is the dad and the light one is the mom, even if none of them are women and one of them is a child.
And people think that not talking to kids about race will keep them from making generalizations...
An interesting paper on this subject (this is a pdf): Children Are Not Colorblind: How Young Children Learn Race