The theme of invasion of privacy is illustrated by the obvious measures taken by the family and their neighbors to keep out undesirables, like thieves and rioters, but the theme is also addressed by the ironic twist that makes it clear that "protectors" are also invaders of privacy.
While the neighborhood is threatened by the invasion of criminal intruders, the agencies purporting to protect against these advertise themselves with small plaques placed on the products of their services. For example, the plaque on the protective wall the family finally chooses reads: "DRAGON'S TEETH The People For Total Security."
In an ironic and horrible twist at the story's end, the theme of invasion of privacy turns readers' attention to those in the business of protecting privacy when, in a shocking surprise, the son is killed in an innocent game of "the Prince who braves the terrible thicket of thorns to enter the palace and kiss the Sleeping Beauty back to life." The worst realization comes to reader and happy family when it is seen that the "DRAGON'S TEETH ... Total Security" has wickedly invaded their privacy through death bred from manipulation based upon fostered hysteria. This is shown symbolically, where the personified alarm is "wailing" against the "screams" of the little boy, in the final sentence of the story:
The man and his wife burst wildly into the garden and ... the alarm set up a wailing against the screams while the bleeding mass of the little boy was hacked out of the security coils ....
It is this part of the thematic message of invasion of privacy that has significant relevance to the situation abroad in the world today.
The invasion of privacy represents the major family's major fear in "Once Upon a Time."
The family fears that the outside world will invade their private world of happiness. This becomes the family's primary motivation:
For when they began to live happily ever after they were warned, by that wise old witch, the husband's mother, not to take on anyone off the street. They were inscribed in a medical benefit society, their pet dog was licensed, they were insured against fire, flood damage and theft, and subscribed to the local Neighborhood Watch, which supplied them with a plaque for their gates lettered YOU HAVE BEEN WARNED over the silhouette of a would-be intruder.
The family fears that the outside world is going to invade their privacy. It is for this reason that they protect themselves by building the gates that have the sign in front of it. The invasion of privacy looms large in the family's mind. It shows that they are afraid of the world.
This fear is seen throughout the story. The worry of the "riots outside the city" and their potential entry into the suburbs is another instance where the invasion of privacy dominates the family's thoughts. This is further enhanced with the building of the wall, the wiring of the burglar alarm throughout the house, and the building of "razor-bladed coils all round the walls of the house where the husband and wife and little boy and pet dog and cat were living happily ever after." In each of these settings, the family cannot trust the outside world. They believe that this world is going to invade or intrude on their private world of happiness and contentment.