Answer:
What you can infer about what Gordimer leaves unstated at the end of the story is the family`s reaction after the death of the couple`s little boy. She didn`t write about the parent`s feelings while they carried the bleeding body of their son into their house. At this point, you can also infer that the parents realized how fool they were and they recognized their mistake of trying to overprotect themselves from an unknown force which threatened their safety.
It relates to the statement about the family living "happily ever after" by demostrating that no one really lives happily ever after, since everybody will always go through one hardship after the other, but no matter what, you have to see the positive side of everything.