While Harper Lee's choice of a child narrator is not original--Dickens often relied on child narrators, and Twain's most famous one is a precocious boy (Huck Finn)--it is certainly a brilliant decision because it perfectly fits Lee's purpose for writing the novel. Her first person narrative, especially her use of Scout as the narrator, accomplishes several key goals for Lee.
1. Lee is discussing issues--racism, prejudice, social class strife--that were inflamatory during the time that Mockingbird was published. She knows that by setting the book in the Deep South, which in 1960 was rife with racial tension, that she is going to stir up controversy. However, by choosing an innocent, intelligent, and likable girl to discuss these "grown-up" issues, Lee widens her audience. If a child such as Scout can see the horrible effects of racism, some adults might be forced to look at their beliefs in a different way. Likewise, the child narrator, with all her precociousness, makes thinking about the issue and our country's mistakes a tad more palatable. Lee's choice, in this sense, is very close to Twain's reasoning for choosing Huck as his narrator. Readers, at first, did not like a smart-mouthed boy voicing adult opinions about their social views--but the character of Huck is what makes the book still enjoyable and powerful today. The same can be said of Scout.
2. Secondly, Lee's novel contains autobiographical elements, and I think that she sees herself as Scout. Lee grew up with her lawyer father and had a very close relationship with him. By all accounts, she was a forward-thinking child--like Scout--and idolized her father. She even followed in his footsteps and went to law school. Her choice of Scout as the narrator gives Lee an opportunity to comment on many aspects of her Deep Southern, small-town background and questions that she had as a child or reflections that she started to experience as an adult. Scout's view of her town is most likely very similar to Lee's view of her town (Monroe, Alabama).
3. Finally, by using a child who matures through the course of the novel, Lee demonstrates what mature views are on the important issues of her day. Lee can have a child discuss issues like racial language, hypocritical teachers, and gross injustice because a child has a right to be innocent about these elements and his/her maturation process would include a character forming an opinion about why these struggles exist and what the solution is to them. Mockingbird is a Bildungsroman, a work that shows the development or maturation of an individual, usually a child, so Lee hopes that her choice of structure and narrator will cause her audience to mature alongside Scout.