During childhood one only knows life through the medium of family. One child lives in a certain kind of house, he has a certain number of family members, he has a father with a certain job, and there may be certain customary activities that they observe. Life is simplistic. Young Bruno knows only these basic facts about his friends; he is not concerned with anything else because his focus is merely upon his interaction with the other boys. When they talk, it is probably about their teachers, classmates, friends and what games they will play. They are innocent and unaware of what historical moments are being played out in their time.
About the other boys' fathers, Bruno simply knows from hearing them tell of their fathers at school that Karl's father is a green grocer, which Bruno knows is true "because he ran the greengrocer's shop in the centre of town"; Daniel's father is a teacher, which Bruno knows is true "because he taught the big boys it was always wise to stay clear of"; and Martin's father is a chef, which Bruno also knows since whenever he comes to pick up Martin he "always wore a white smock and a tartan apron, as if he just stepped out of his kitchen."
Because Bruno only knows superficial details about his friends and nothing outside his little world, he is entirely innocent; in fact, he does not even know what his military father does other than that he is "a man to watch" and the "Fury has big things in mind for him"--words he must repeat when asked by the other boys what his father does. His mother, too, reiterates that his father's job is "very important," and says little more.