Tools of Characterization
Abner's violent actions, like burning barns, ruining rugs, hitting family members, dominate our idea of his character. Even if we feel a little pleasure in seeing him track poop on the rug of a man who was probably a slave owner, we recognize that his actions aren't productive, and probably won't result in positive change for anyone.
While Abner's actions are pretty consistent, Sarty's actions change depending on whether he is trying to please his father or himself. For example, when Sarty tells the second Justice of the Peace that his father didn't burn de Spain's barn (which was true though Sarty didn't know it) he is acting like a liar, like the son his father wants him to be. When Sarty decides to act according to his own principles he risks everything to tell de Spain the truth. This characterizes him as a person who sees his duty to himself and society as greater than his duty to his family.
In the very first paragraph we learn that Sarty can't read. We can be fairly sure that no member of the Snopes family has seen the inside of a schoolhouse. In those days even a basic education was difficult to come by for those living in poverty. For one thing, the children were expected to work full time alongside the parents. For another thing education wasn't considered to be a necessity for members of the working class, but rather a privilege for the wealthy classes. As you probably know, the more education you have, the more options you have. The lack of education of Sarty's mother, aunt, and sisters in particular further characterizes them as women with few options.
The Snopes family's appearance characterizes them as poor. Their wagon is filled with broken possessions. Sarty's pants are too small, and his shirt is so thin it tears when de Spain's servant grabs onto it (7, 106). His sister's wear "cheap ribbons" (35). They live in a two-room house. They can't hide their poverty. After the little boy hits Sarty and knocks him to the ground, Sarty's mother wants him to "wipe some of the blood off before hit [it] dries" (24). She seems to be concerned with the appearance of blood on her little boys head, which is only natural. It's embarrassing for a parent to be seen with a bloody child. If the child appears bloody, then the parent probably isn't doing his or her job. The people who see Sarty with blood on him will assume that his parents are to blame. This moment gives us a glimpse of Lennie Snopes's shame at her inability to appear or be a mother who can protect her son.