Neither Bruno nor shmuel really know going on at the concentration camp with that and what allows them to keep their innocence

Bruno is sheltered by his family. He lives in the midst of a death camp, but his mother and father keep him innocent of what happens on the other side of the fence. Shmuel is a different story. He also shelters Bruno. Shmuel hints at things and reveals what happened to his home and family but goes no further. There are a multitude of boys at the camp, but they are kept isolated. This would account for Shmuel's remaining innocence.