This is a very difficult question. The answer is not necessarily "out there" to be found, as Faulkner's use of italics in this novel are not entirely consistent.
We can see some relationship between uses of the italics across the whole novel, where on many occassions the italics represent moments of memory and/or private thought. In Darl's case, this is not true, however, and italics in his sections tend to be connected to danger and to Jewel.
The first temptation is to interpret italics in Darl's sections as episodes of his "connection" with his family. Darl has out-of-body experiences and is aware of things that he has not physically witnessed. It might be a good way to read the italics in his sections as moments where this connection is active.
This interpretation fails to explain the italics used when the family is attempting to cross the creek.
Perhaps the most likely explanation is a stylistic one. In Darl's sections, the use of italics functions as a less impressionistic mode of prose than is used throughout the rest of his sections. So the italics may simply be a way to contrast sentence styles without compromising the larger consistency of the fabric of the prose.