What is the significance of Bruno wearing a ringmaster's costume at the party in chap 8?

This is an interesting point. Behind the scenes at this party, there is something very insidious going on. Bruno's dad is going to become commander at a concentration camp. The death of millions of Jews is being discussed. Yet the kids are still putting on a play. That was what he was wearing that night for the play, "Gretel and Grandmother always put on!" That demonstrated Bruno's naiveté because he doesn't understand and wasn't aware of the fact that everyone was mad and having a very intense conversation.

Grandfather supports his son and thinks he looks admirable in his uniform; he is proud. Grandmother, on the other hand, is ashamed of her son. She does not support his position in the Third Reich, his pride in his uniforn, or the actions they are taking against the Jews.