To my mind, the powerful climax of this great novel occurs in Chapter 15 of the novel, after Kenny has fought with the Wool Pooh twice, firstly in the water at Collier's Landing and secondly in the church that suffered the bombing. On his return to home, Kenny is understandably completely traumatised, far more than Byron and Joetta are, and finds that he needs to be silent and withdraw from everyone to deal with what he has observed. His parents and Byron are incredibly concerned for him, and Byron takes it upon himself to get his younger brother better.
One day he insists that Kenny accompanies him to the upstairs bathroom to show him a hair that is growing out of his chin that he is incredibly proud of. Kenny then takes a look at his own face in the mirror, trying to see his own moustache and how long it has grown:
Maybe it was because I hadn't looked in the mirror for a long time, but as soon as I saw myself with my lazy eye still being lazy and my face looking so sad I slammed my eyes shut and started crying. I even fell off the toilet. Byron caught me and set me on the floor.
This then is the climax of the novel, as Kenny is finally able to emotionally express all the terrible experiences that he has been through and finds release through crying and letting out his suppressed emotions.
Yes - I agree that is the climax of the novel. The climax is also the fulfillment of the conflict. The main conflict is that Kenny's brother By can't behave. That is why they took the trip in the first place. The conflict is resolved during the climax when By helps Kenny come out of his depression/shock. Thus By learns to appreciate his family By is a very interesting character. He went through an even stronger emotional change than Kenny, the main character.