Shirley doesn't tell the story in her own words, but instead an unknown presence does it for her. This third-person perspective allows us to view the events in Shirley's life from a bit of distance, taking in her whole path instead of getting stuck inside the details of her mind. Which isn't to say that we're never in her mind, because we totally are. The narrator dips into Shirley's thoughts to see what's going on in there from time to time, and these visits help us feel close to her as our main character. But we're never there for too long, so we never lose track of the bigger picture.
Question: Since Shirley is ten, and doesn't speak English for part of the book, how would this story be different if it were instead written in the first person, with Shirley herself taking the lead?