Narm: Rubin's death in the movie, which was more of an in-universe Epic Fail than anything else. It's the cartoonish "Doh!" sound he makes when he falls that does it.
Squick: Old Dan's injuries. His stomach is torn open by a mountain lion. He survives and tries to follow Billy home, only to have his intestines spill out of the open wound and tangle in some brush. A sobbing Billy is forced to clean dirt and leaves off his faithful pet's intestines and stuff them back into the poor dog's body cavity. This book was written for ten-year-olds.
A close runner-up is Rubin's death, which is shockingly graphic and prolonged for a children's book: while threatening Billy and his dogs with a hatchet, Rubin trips and impales himself on the blade. He begs Billy to take the hatchet out. Billy obliges, wiggling the blade from the other boy's stomach. A "bubble of blood" works its way out of Rubin's mouth. It bursts, and Rubin drops dead.