In "Barn Burning" Faulkner uses third-person limited omniscient point of view that focuses on Sarty's dilemma. In this way, the reader is placed in Sarty's position, torn between family and social allegiance.
He also uses a stream-of-consciousness (long, winding, sentences with multiple perspectives) that show how Sarty is confused, disoriented, and conflicted between his outlaw father and social justice.
Faulkner really can't use Sarty as a narrator, because he's too young to understand the complexity that the story demands. Therefore, Faulkner filters Sarty's thinking through an older authorial voice, who, of course, is more experienced with elements of clannishness, Gothicism, Southern values and culture.
Enotes says it best:
Faulkner was a perspectivist: That is to say he liked to tell a story from some particular point of view—or sometimes, as in the novels, from many divergent points of view, each with its own insistent emphasis. ‘‘Barn Burning’’ offers a fairly controlled example of the application of perspectivism. Faulkner tells his story primarily from the point of view of young Sarty, a ten-year-old boy. This requires that Faulkner gives us the raw reportage of scene and event that an illiterate ten-year-old would give us, if he could. Thus, Sarty sees the pictures on the labels of the goods in the general store but cannot understand the lettering; adults loom over him, so that he feels dwarfed by them; and he struggles with moral and intellectual categories, as when he can only see Mr. Harris as an "enemy." There are few departures from this strict perspectivism, but they are telling, as when, in the penultimate paragraph of the tale, an omniscient narrator divulges the truth about Ab's behavior as a soldier during the Civil War. But even this is a calculated feature of Faulkner's style: the breaking-in of the omniscient narrator is another way of fracturing the continuity of the narrative, of reminding readers that there are many perspectives, including a transcendental one in which all facts are known to the author. One further note about the story's confined perspective: Sharing Sarty's immediate impressions and judgments forges a strong bond between the boy and the reader.