Just to lighten the mood on the thread a little -
I remember taking my three-year-old on the bus to nursery, when she saw her first black person - there were virtually no black people where we lived at that time - an old gentleman sitting a few seats away.
Having stared at him for some time, my little poppet advised me, in a voice loud enough for the entire lower deck to hear - "Daddy, that man's got BLACK SKIN!!!" and I replied (rather more quietly!) that he was a black man.
She seemed satisfied with the response, but having examined him a little more closely, she advised in a similar foghorn level stage whisper "He's got BLACK EARS!!!"
The gentleman in question smiled indulgently, and I smiled back at him.
I did not see the need to upbraid my daughter for transgressing any perceived racial sensitivity lines - as others have advised, three-year-olds say what they see.