In "Harrison Bergeron," why do Mr. and Mrs. Bergeron seem so calm even though their son is in jail at the story's beginning?
Harrison's parents do not bemoan his imprisonment or even his demise at the story's end for several reasons.
First, they physically and mentally cannot bemoan anything. Vonnegut writes that
"George and Hazel couldn’t think about it very hard. Hazel had a perfectly average intelligence, which meant she couldn’t think about anything except in short bursts. And George, while his intelligence was way above normal, had a little mental handicap radio in his ear" (Paragraph 3).
Hazel cannot concentrate on her son's disappearance because she lacks the mental capacity to do so, and if George tries to remember or ponder anything, his ears are filled by the government with loud noises which drown out any deep thinking.
Secondly, in a "perfectly average" society, Vonnegut stresses that everyone is controlled by the government; so even though Harrison is the Bergerons' son, they go along with whatever the authoritarian regime wants/requires. They accept that because of Harrison's many exceptional qualities he must be exceptionally handicapped to be equal with no unfair advantages in an average society.