Mother's reaction clearly demonstrates fear. As an adult and with her husband the commandant of a camp, she knows what Hitler and the Nazi party really stand for and these are not values she wants her twelve year old daughter to embody. It's also a realisation of just how powerful the Nazi party have become, that they have been able to influence children through the Hitler Youth and through education. On some level, Mother is also angry. Isolated from most of the world, Gretel and Bruno only have limited influences and Mother observes that the indoctrination of the Nazi agenda and ideas comes from the children's tutor.