It all came down to what box the family had checked for their kid's race- so much so that a child who I spent the whole year thinking was German based on looks of him, and mother and his name was considered the one diversity boy in next year's class because his father is mixed race. I am not saying that this child does not bring diversity to the school, but if you never see the dad or talk about what makes this child different, no one would ever know. In a class of 18 kids this one boy was, in my opinion, the token non-white; except for the fact that he is (at least in skin tone) whiter than me.
Little kids see differences in concrete ways: your hair is curly, mine is straight; "I have a penis with a ring, you don't have a ring on your penis"; "Regina you are spotted like my mommy, and I am all over brown like my daddy". Yes I think children need to be exposed to people of other colors than themselves so they can see these differences and not be afraid of the "other", but I also think our differences (and similarities) lie more within our cultural frameworks and upbringings than in the color of our skin. Little kids are capable of having discussions about culture; how is your family the same as mine? how are they different? And here's a hint its not just the color of our skin that sets us apart or brings us together.
I am fully aware that at some point my own children will ask me, (or my sister, or the child) why my sister's kid is a different color than the rest of the family, and the answer will be because aunt liz adopted him from a different place than our family is from. It's a simple answer (and one that will grow more complex as the children grow older) but really isn't that what skin color comes down to- geographic location of your ancestors? I know that this simple explanation doesn't address the values placed on the colors of people's skin but that is sort of the point, but there is a time to address that issue; and its not when a kid is 3.