In my opinion, Harper Lee's use of Scout as narrator of this story is clever and exceptionally effective. Because Scout, as a child, is an innocent, her recognition of evils and moral wrongdoings resonates particularly powerfully with readers. Scout is often unaware of the cruelty and potential for badness present in her fellow Maycomb citizens, but, when she does intervene or become cognizant of a wrongdoing, her reactions to the prejudice, ignorance, and other negative characteristics of many events and people in the novel come purely from a place of goodness and purity.
Due to the fact that she is a child, and especially because she has been raised in an atmosphere where she has been protected from developing poor moral attitudes, etc., Scout is able to view the world as consisting almost completely of areas of black and white; there are very few gray areas in Scout's outlook on the world. Lee was obviously aware that presenting a story that encompasses so many lessons in morality would be most powerful if it was revealed as it might have been seen through the eyes of a child.