Harper Lee uses diction(word choice), characterization, point of view, and mood to create an atmosphere of childhood.
From the start, readers discover through both point of view and diction that the story is being told by a child. Scout demonstrates fear of Boo Radley in the opening chapter as she describes the neighborhood legends about him. The language (about mutilating chickens and his blood-stained hands) is consistent with children who would fear a phantom, not adults who would talk about the hermit who lives down the street. She sees her town and surroundings throughout the first half as a series of obstacles a child experiences, not a social community with the unredeeming flaw of racism. In her perspective, readers notice the proximity of the Radley's to the school, readers understand the classes of the society through the 1st grade classroom, and readers experience the elderly, the middle class, and the Negros through her eyes.
The characterization of Scout, Dill, and Jem all demonstrate type-cast childhood stages and experiences. For example, Scout is the child who perpetually fights society's effort to pigeon-hole her into a mold. Jem is the child dealing with emotional instability due to the early death of his mother and the fact that he is dealing with adolescence. Dill is the abandoned or unwanted child trying to find worth in something.
Finally, the theme of innocence helps create a mood of childhood throughout the work. Although Lee's purpose is to send several messages about very adult issues, when readers experience the innocence of Tom, Boo, and even the children, the brash nature of injustice is given more serious attention. Audiences can relate to a man being treated unfairly, but when seen through the untainted eyes of a child, readers are reminded that morality is still important.