How does Stanley's character change while he is at Camp Green Lake?

At first, Stanley follows the lead of the boys, particularly X-Ray, in an attempt to fit in. This causes him to adopt some of the cruelty that surrounds him as well, and he is dismissive of Zero when Zero asks him to help him learn how to read. Once Zero helps Stanley, however, Stanley feels bad and decides to help Zero read. Through this action the two become friends and Stanley develops more self-confidence. Although Stanley enters Camp Green Lake as a shy and timid boy, he leaves with self-confidence and bravery.

How do Sam's onions help Zero and Stanley?

The onions provide Zero and Stanley with food and medicinal benefits. Zero had a very serious stomach problem after drinking the 100 year old spiced peaches, but the onions make him well again. Eating the onions also gives Zero and Stanley onion-blood, which helps keep them safe from the bites of the yellow-spotted lizards.