When the story begins, Sarty Snopes is awaiting his turn to testify in a legal trial against his father, Ab Snopes, who has been accused of burning down another man's barn. The Justice of the Peace is presiding over his father's case in a local store, and Sarty thinks that he can smell many food items around him as well as the fear and despair and grief of various people on the scene, and, finally, he can feel the "old fierce pull of blood." This would seem to suggest the loyalty he feels toward his father or at least his awareness of his father's expectation of loyalty.
When the Justice first addresses Sarty, asking for his name and making some light of it, the boy "said nothing" but thinks, "Enemy! Enemy!" Despite the Justice's kindly face as well as the fact that he clearly does not want to compel this small boy to choose either his father or his honesty, Sarty thinks of him as an enemy because he could rule against Sarty's father. This is another example of Sarty's loyalty.
Even as it becomes clear that this is not the first barn Ab has burned in order to exact revenge on someone, Sarty tries not to even think such thoughts. He does not know where they are headed but there was "always a house of sorts waiting for them a day or two days [...] away. Likely his father had already arranged to make a crop on another farm before he . . . Again he had to stop himself." Sarty tries not to allow himself to consider his father's guilt, showing his loyalty. He stops his train of thought before it reaches the conclusion that his father did burn down the barn.
That night, his father accuses Sarty of "fixing to tell them" the truth at the trial, and he strikes the boy just as he would strike a mule. Sarty does not talk back or do anything to rebel or even cry; "He just stood there." This seems like loyalty, too.