A disappointment. As many others have pointed out, not a realistic 9-year old and not very well-written.
I know that the 'how' of the holocaust in real life has often been met with "ich habe es nicht gewüsst"
, but even when considering that, the 9-year old main character to me just seems way too naively portrayed. He's the son of someone high-ranked in the SS, but he has no idea what "Jew" is or means, even though his father invites other SS-relevant figures to dinner. The constant 'mishearing' of Fuhrer ('Fury') and Auschwitz ('Out With') is just odd. (This is extra annoying because the book is in English and we're to assume the characters really are speaking German as they identify themselves as German and nothing says otherwise - and I don't think words would be misunderstood that way if it's in their mother language instead of hearing the word wrong because you speak another language.)
The frustration with that character for the same reason did make the ending extra interesting.