What is a single pivotal moment in the moral development of Scout in Harper Lee's To Kill A Mockingbird?

The trial demonstrates Scout's moral development.

Scout grows up considerably over the course of the book.  When the book first begins, she is going into first grade and very immature.  She doesn’t understand the situation when she finds out that her father is defending Tom Robinson.  She thinks Boo Radley is a monster.  She fights with other children at the drop of a hat.  However, in time Scout grows to better understand human nature and develop an adult's moral consciousness through empathy for others.

The moment when you first realize that Scout's moral sense has developed to the point that she understands human nature is during the trial.  Scout shows that she is thinking about her father, her brother, and Mayella.  This is a sign of maturity.  Earlier on, Atticus tried to get Scout to see things from others’ points of view.  Scout begins to do this.

The first example is when a group of Cunninghams try to lynch Tom Robinson while Atticus is the only one there to stop them.  During the lynch mob incident, Scout walks into the middle of a mob of angry men and tries to strike up a conversation with Walter Cunningham’s father, based on what little common ground she can find from what little she knows about him, and the fact that she goes to school with his son.

Atticus had said it was the polite thing to talk to people about what they were interested in, not about what you were interested in. Mr. Cunningham displayed no interest in his son, so I tackled his entailment once more in a last-ditch effort to make him feel at home. (Ch. 15)

In this way, Scout diffused a very serious situation.  She did not completely understand what she did nor how she did it, but she did it based on a very grown-up impulse to try to understand someone she barely knew.  She was doing what Atticus taught her, albeit inexpertly.  Despite this, or maybe because of it, she succeeded.  Scout broke up the crowd, because they could no longer go through with anything after that.  As Atticus said, “it took an eight-year-old child to bring ‘em to their senses” (Ch. 16).  Scout taught a moral lesson to a group of men.

Scout also showed that she appreciated what others went through when she listened to Mayella testify.  You would think that Mayella would be her enemy, since she was the one who accused Tom Robinson of rape and Atticus was defending him.  As Scout listened to Mayella testify and her father’s cross-examination, however, she put herself in Mayella’s place.

As Tom Robinson gave his testimony, it came to me that Mayella Ewell must have been the loneliest person in the world. She was even lonelier than Boo Radley, who had not been out of the house in twenty-five years. (Ch. 19)

Scout also shows compassion for Jem during and after the trial as she realizes that he expects Atticus to win and he is heartbroken when he doesn't.  She watches her father very closely too, and feels for him every step of the way.    She doesn't understand everything that happens, but has empathy and an emotional connection.

The trial is pivotal because it shows Scout what the world is really like.  It requires her to show compassion for Walter Cunningham, Mayella Ewell, Atticus, and her brother.  The trial affected all of their lives, but for Scout it demonstrated that she was really growing up.