When was Scout a moral coward?

Moral cowardice is the avoidance of taking a principled stand for something based on the reception you will get from others. It assumes that the person has strong principles but chooses not to stand up for them because they fear the reaction of other people. At six years old, Scout may not have a highly developed sense of principles or a deep sense of morality. We actually see a number of times that she acts with great moral courage for her age, including saying what she thinks is right on the first day of school.
But there are times when she does things that she knows she isn’t supposed to do. One clear example of this is when she joins Jem and Dill in sneaking onto the Radley property at night. Atticus has clearly dissuaded the children from bothering the Radleys with their games or trying to sneak notes onto the porch. But Jem still wants to sneak into their yard and peek in the house. Though it’s not explicitly said, Scout knows Atticus wouldn’t approve. Jem goads her with the idea that she is turning into a girl, and this feeling of being left out pushes her to join them, despite her misgivings.