How does Harper Lee use Scout's innocence as a vehicle to explain Atticus' attitude toward African-Americans?

Harper Lee relies on Scout's innocence, curiosity, and precociousness to illustrate Atticus' unbiased treatment of African-Americans.

First, Atticus entrusts Scout and Jem to Calpurnia.  She is not simply a cook or servant.  Calpurnia is a vital part of the Finch family--she teaches Scout morals and ethics; she watches over the children when Atticus has to be away.  Because of Calpurnia's "mother-child" relationship with Scout (encouraged by Atticus), Scout does not see race when she looks at others.

Scout's curiosity and perceptiveness cause her to ask her father questions not only about the trial but also about puzzling incidents that she witnesses.  Atticus' replies to Scout's questions portray Harper Lee's beliefs, too, on race issues.  When Jem and Scout ask their father about the trial because they simply cannot believe that Tom Robinson was found guilty after Atticus' masterful handling of the case, he has opportunity to express how justice should work versus how it does work in Maycomb because of racial prejudice.