The Boy In the Striped Pajamas

I am currently reading The Boy in the Striped Pajamas by John Boyne. I am not far into the book but from hints and hearing about it, I know it involves the Holocaust in the story as important to the plot. The story is being told from the standpoint of a nine-year-old boy named Bruno. I think it is interesting why the author chose to tell the story from a 9 year old who is not mature and doesn't understand the world. This is not like The Book Thief because in that book, Liesel at least has an understanding of the way things are in the world. This seems like it's more difficult to write from the standpoint of someone so ignorant on a topic so complex. In the book thief, at least, Death was wise and old. Here, it is first person with someone who is young and ignorant. I wonder what caused the writer to choose this way to write the book.