Why has no one told Bruno the reason for the concentration camp, and why does Bruno's family not realize that he is gone and, then, see him among...

Bruno and his sister are probably not informed about the concentration camp because they would be horrified by the knowledge of the truth and, then, beg to return to Berlin. And, since the camp is a considerable distance from the house, the parents try to keep the children ignorant of its purpose. From such a distance, then, the Commandant, who spends long hours in his office, or the mother would not recognize Bruno in the garb of the prisoners even if they did look out at the camp. 

In addition, it is important to realize that The Boy in the Striped Pajamas is a fictitious narrative based upon historical events, not a documentary. For one thing, most Jewish children were killed by the Gestapo because they could not work and serve some useful purpose. Almost certainly, Shmuel would never have come to the house of the commandant to help in the kitchen, but this incident serves the author's purpose of displaying how Bruno's fear causes him to betray Shmuel. So, if there are minor credibility gaps in the novel, they are simply because the author creates his own story that has specific themes and intent.

One of these themes is that the Jews persecuted in Germany were human beings, just as Bruno's family is composed of human beings. That the Nazi soldiers do not recognize any difference between the two boys in their "striped pajamas" and little hats on their shaved heads indicates that some of the propaganda about the ugliness of the Jews must be exaggerated.

Shmuel blinked and shook his head. It was quite extraordinary. If it weren't for the fact that Bruno was nowhere near as skinny as the boys on his side of the fence, and not quite so pale either, it would have been difficult to tell them apart. It was almost (Shmuel thought) as if they were all exactly the same really.

Bruno's character stands for human compassion for others. In order to exemplify this theme with his character, John Boyne has clearly generated some credibility gaps in his narrative, but his literary purpose is, nevertheless, served in this work of fiction.