Why does the author use plural first person narration for this section of the book?

Julie Otsuka uses the first person plural voice in narrating the events of chapter 4 of When the Emperor was Divine. Doing so allows her to tell the stories of so many Japanese-Americans, to convey more effectively the full range of experiences during the period of internment. Because this chapter is told from the perspective of more than one person, Otsuka is able to tell a much bigger story about the family's experience of returning home to California, to a life of poverty, racism, and chronic uncertainty.
The use of the first person plural in chapter 4 needs to be placed in the context of the overall structure of the book. Prior to this chapter the story has moved from the mother's perspective to the daughter's, as the action shifts to the internment camp in Utah, then to the son's for the remainder of the family's internment. As we engage with the collective consciousness of the children, it is as if we're being taken deeper into the family's confidence, deeper into their collective experience of the nightmare world into which they have been thrust. The combined voices of the boy and girl synthesize the experiences hitherto gradually revealed in the previous chapters. Their recollection of the harshness and insecurity of post-internment life in California is a suitably collective one, reflecting the felt experience of thousands of other Japanese-Americans.