What do Bruno and Shmuel tell us about friendship through their unusual relationship?

The relationship between Bruno and Shmuel is considered usual due to the fact the Bruno is German and according to Nazi ideology, a member of the "Master Race" While Shmuel is a Jew of Polish ancestry. Common knowledge during the Holocaust were that Nazi's treated Jews notoriously as "sub-human" So such a friendship between Bruno and Shmuel would never be allowed under normal circumstances. The fact that Bruno is ignorant of the position his father holds, that he thinks the concentration camp member are "Farmers." Adds to the childish simplicity of their budding friendship. So clearly Bruno doesn't yet share the belief that Shmuel should be treated as the "enemy." Bruno simply sees Shmuel as another boy around his age that he wants to play and become friends with.
So through Bruno's ignorance, it teaches the readers that a person's creed, nationality or religion, shouldn't define how that person is treated, nor deter you from pursuing and forming friendships with them.

Bruno and Shmuel's friendship is unusual because of the barrier between them. Shmuel is a Jew in a concentration camp, while Bruno is the son of the commandant at the camp. Bruno is completely oblivious to Shmuel's situation and the daily hardships he endures. Bruno doesn't know that he is supposed to hate Shmuel. They become friends despite their circumstances, which shows the reader that despite appearances, religious beliefs, and circumstances, it is ok to befriend someone who is different.

Bruno and Shmuel's strange relationship tells us that friendship—true friendship, at any rate—rises above differences of race, culture, and nationality. Shmuel is a member of a persecuted minority; Bruno's people are the ones doing the persecuting. Not only that, but Bruno's own father plays an important role in the campaign of genocide being carried out against Shmuel and countless other Jews across Europe. On the face of it, then, Bruno the German and Shmuel the Polish Jew are the last people on earth you'd expect to forge any kind of friendship. That they are able to do so indicates just how artificial the various man-made barriers routinely used to divide people are. The fence that separates Bruno from Shmuel is a metaphor for one such artificial barrier, that of race.