My youngest child's best friend is an adorable little black boy named Tre Kwan. They are two of kind, and soooo cute together!
I feel fortunate that my children go to school with such a diverse group of kids and learned acceptance first hand. No amount of books I could buy (heh, and I have A LOT of cultural diversity kids' books!) could teach them this lesson the way life has.
I do think that, for young children, racism IS learned, although I don't think parents always teach it intentionally.
It's like when a child first becomes aware of an obese person, and, out of curiosity about the physical difference, begins asking questions. That's their natural reaction. What's NOT their natural reaction, and what is definitely learned, is a snotty or hateful reaction like, "Look at the fat lady! She's a cow!" (I've heard a small child...4 or 5...say this about an obese lady in a store before).
Children are curious about differences, but when they overhear judgments and negative comments, they WILL pick up on that. I think it's important to indulge their curiosity (and a man missing an arm or a woman with a birth mark covering her cheek, or anyone who looks "different" or unusual to child, will typically not take offense to a child's sincere questions).
Children should be allowed to talk about and question differences. It's up to us parents, though, to make good use of those opportunities and teach our children that different doesn't mean bad.
