Bruno's relationship with his mother can reasonably be described as difficult. This is largely because, in the family dynamic, she is on the side of her husband, acting as his loyal lieutenant when it comes to instilling the children with discipline and Nazi ideology. That's not to say she isn't a loving mother to Bruno. It's just that the normal mother/son relationship has been complicated and distorted, like everything else, by Ralf's new post as Commandant of Auschwitz.
Though very naive about the war and about the horrors of the concentration camps, Bruno is still an inquisitive young boy and wants to find out more about his surroundings. However, his mother expressly refuses to discuss anything that might compromise Bruno's innocence. In all probability, she thinks she's doing the right thing by protecting her son from the harsh realities of the world in which they live. However, Bruno's too young to understand this and resents his mother's stern refusal to answer difficult questions.