The main adavantage Harper Lee gains from using Scout, a 6-8 year old girl, as a narrator, is that her perspective reamins untainted by society's prejudices. Many other people in Maycomb, especially the adults, would not have given such an unbiased report of what was happening with Tom Robinson. Also, Scout's misunderstandings can be a source of humor.
The main disadvantage is that, as a young child, Scout doesn't understand a lot of what's going on around her. A reader needs to read between the lines often to understand what is going on.
One advantage is that Scout is a girl and says and thinks things that other girls can emphisize with. The flip side is that the boys can obviously not emphisize with everything. Another advantage to using Scout has a lisence to ask more questions. Hope that helps!