Biases are ingrained at an early age. In this example, the grandfather tries to hone in the message that there are no nice Jews; the objective of the Jews is to become the ruler of humanity- they are not creative but destructive.
The Nazis assure the men that they are going into a place for a simple shower. Once all the men strip down and are confined in the "shower" space, the Nazis fill the space with lethal gas and wait for them to die.
Bruno finds new friends to play with on the other side of the fence, and thinks they are farmers. In reality, those are prisoners of a Nazi concentration camp, and his mother struggles with how to tell him that.