Not a very easy question to answer because Harper Lee has taken on the job of presenting some very serious adult issues through the eyes and mind of a child just starting school. Scout therefore has to be wise beyond her years in one sense but also has to be believable as a rather tomboyish little girl. To my mind mind the author is successful most of the time as we, the adult readers, are able to interpret the 'innocent' evidence that Scout gives us and see it for what it really is. However, Harper Lee has to resort to overheard conversations a number of times in the novel and this runs the risk of portraying Scout as being a little sneaky and less innocent than we need her to be in order for her to stay attractive to the reader. In general I would say that this danger is skilfully avoided by the author.
As readers, we are compelled to trust Scout's retelling of the events in the novel because we come to know her character as truthful, innocent, and idealistic.
Scout is truthful to a fault--she often gets in trouble for saying the truth out loud at the wrong time, such as when she evaluates Walter Cunningham's table manners. We also view the trial through her innocent eyes. As she comes to understand the racism and other social problems in Maycomb, we understand with her. Finally, Scout is highly intelligent and has an innate sense of right and wrong, and does not care much what people think of her. These qualities make her an exceptional narrator because she does not try to make herself look better than she really is.