Think about Scout's fight with Francis, and Atticus‘s message to Uncle Jack about children being able to spot an invasion quicker than adults. How do these two elements connect to the value of having scout a child be the narrator the story?

This is Scout's narration.... we are supposed to see how innocence is lost.... how the adult world slowly impedes on the lives of children. Francis insulted Atticus, using the words of adults..... things he had heard. His comment and behavior exposed the way adults influence the words and actions of children.

Scout, however, reacted without thought.... standing up for her father and the family name. Uncle Jack, havving no children of his own, was looking at her behavior.... not the cause of her behavior. For him, it didn't matter what was said. The only things that mattered was that Scout gave her cousin a beating.

Well, first of all, you have to remember that although Scout is the narrator and she is recounting her childhood (being a bildungsroman novel) Scout is actually an adult now. Therefore perhaps the overall significance of this is that Scout is preaching what her father taught her.

Yes, in a large way, the novel is about innocence, if that is what childhood is. So, in a way, children being able to "spot an invasion quicker than adults" is a byproduct of them caring only about the things they have learnt to care about so far: family being a big one in Scout's case. Adults, even Atticus, are 'grown-up' already and have things such as work and friends and general expectations of society to worry about. -E.g: if somebody did insult a friend or family member of yours, I'd hope you wouldn't be so quick to physically attack them - because you have the knowledge and experience to care about other things such as consequences and 'taking a walk in another man's shoes'.

In such a way, this extract-bit is a good example of how we witness Scout as a young child, to how she would feel about it as an adult. -Nevertheless, you wouldn't necessarily determine young Scout's behaviour as 'preposterous', because children scrap and fight, and it would be unusual if Scout had gone up to Francis and asked him nicely and calmly to stop. --The main lesson how Scout grows up with the help of Atticus.