Are kids just the mini-me versions of the adults they will become, or is something substantial lost—or gained—in the transition to adulthood? And how does that process work, anyhow? To Kill a Mockingbird shows a child's perspective on adult events, and suggests that while children aren't just adults in miniature, they also aren't what adults imagine or misremember children to be. You gain a little and you lose a little as you grow up, and some of the abilities that disappear—like fairness, compassion, and a critical way of looking at the world—are well worth trying to keep.
The children have an innocent perspective that reveals what the adults don't see.
The novel associates children with fairness to suggest that a sense of justice is innate, not learned, and therefore adults must have learned to be unjust.