Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Fearful; Determined
Given that The Wild Children's narrator stays firmly planted inside Alex's brain for the bulk of the novel, the book's tone mirrors Alex's emotions about his experiences.
As it turns out, Alex is scared most of the time. This is pretty understandable, to be honest, as Alex is thrust into an unfamiliar world without a single friendly face to help him. This leads him to describe things in a way that highlights their fearsomeness, such as when he compares his experience to "being clawed by wolves and left bleeding in the snow, a nightmare of his childhood" (2.6). It might be a little melodramatic, but it's an accurate reflection of his feelings.
As Alex's confidence grows, however, we see the book's tone shift slightly. Although there are still plenty of moments when Alex is straight-up terrified, the novel takes on a distinctly determined tone, such as when Alex describes feeling "joy," "for the first time in the months since he left his home" (8.41) when the crew heads south. This a reflection of Alex's growing comfort level with his new friends, as well as his ever-increasing maturity. Ultimately, these two distinct tones are an accurate reflection of Alex's wide-ranging emotions over the course of his ordeal.