why do the other children hate Loung

I'm not sure what children you're referring to, or which section of the novel this question pertains to. Loung's family is hated because they've been westernized...... their family was wealthy, they had nice things, they benefited from Capitalistic practices, something abhored by Communists.

All others are forced to pick up and move when the soldiers say so. The base people will train us to be hard workers and teach us to have pride in our country. Only then will we be worthy to call ourselves Khmer. I cannot comprehend why they hate me or why capitalists must be killed…When I ask Kim what a capitalist is, he tells me it is someone who is from the city. He says the Khmer Rouge government views science, technology, and anything mechanical as evil and therefore must be destroyed. The Angkar says the ownership of cars and electronics such as watches, clocks, and televisions created a deep class division between the rich and the poor” (p. 58).