Lee begins To Kill a Mockingbird with an epigraph (a brief quotation placed at the beginning of a book or chapter) by Charles Lamb: "Lawyers, I suppose, were children once." That she chose this epigraph is interesting on several levels.
A good part of this story's brilliance lies in the fact that it's told from a child's point-of-view. Through Scout's eyes, Lee is able to present the story objectively. By having an innocent little girl make racial remarks and regard people of color in a way consistent with the community, Lee provides an objective view of the situation. As a child, Scout can make observations that an adult would avoid or sugarcoat. Readers, too, are likely to be forgiving of a child's perception, whereas they would find an adult who makes these remarks offensive.
Much of Harper Lee is in the character of Scout. Lee's father was an attorney, as is Scout's. Importantly, Lee herself studied law. Because Scout's personality is loosely autobiographical, the epigraph makes sense. Lee proves through the telling of the story that she was also once a child.
Also significant in understanding the epigraph is Atticus' answer to Jem's question of how a jury could convict Tom Robinson when he's obviously innocent: "'They've done it before and they did it tonight and they'll do it again and when they do it — it seems that only children weep.'" At various points in the story, Jem expresses his desire to become a lawyer, following in his father's footsteps. The lessons he learns during the course of the story will ultimately shape not only the kind of lawyer he will be, but also the kind of man he will become. Readers see this future lawyer as a child first.