Because of the handicaps that George has, his reaction to his son's murder is brief, and because of Hazel's "low" intelligence and short attention span, hers is also underwhelming. Their son, Harrison, has been gone for a long time, imprisoned by the Handicapper General and her thugs. So when they see him on television, it is for the first time in months. You would imagine immediate recognition, and joy at seeing him, but, George's recognition is blasted away by a loud sound in his head. After Harrison is shot, George went "into the kitchen for a can of beer." No reaction; the death was probably quickly erased by a loud "handicap" of a distraction in his ear; in the brief span of the story, these handicaps have already happened multiple times, and we realize that George can barely think straight at all, about anything. That includes his son. So, his reaction? Nothing, because the handicap that he has assures he can't dwell on anything or remember it for too long.
Hazel has a more physical reaction; she starts crying. She is, momentarily, deeply distressed by what has occurred, but, because of her level of intelligence and memory, she quickly forgets what happened. Her husband sees the tears and asks what is wrong, but she can't remember. The closest she gets is remembering that "something real sad on television."
In the Bergeron's society, handicaps assure that no one ever feels anything negative--sadness, grief, misery, jealousy, or low self-esteem. So, when Harrison is gunned down in front of them on television, his own parents feel only a momentary pain before forgetting. I hope that those thoughts help; good luck!