What is symbolic about the narrator's house resting on an insecure foundation in "Once Upon a Time"?
The thematic concern of "Once Upon a Time" is security. At the mother-in-law's behest, ever-growing security measures are put in place. These security measures begin with medical benefits and insurance policies and escalate from a "You Have Been Warned" sign on the front gate to iron bars on the windows to more bricks (a Christmas, no less) for a higher fence. The final security measure is jagged metal and coiled wire atop the high brick wall.
All these security measures result in the son, who has just been read Sleeping Beauty, envisions the metal and wire as his own obstacle to his sleeping princess. In this make-believe world, he scales the wall intending to make it victoriously to the other side. Instead, he is ensnared and impaled by the metal and wire and loses his life. The thematic concerns are brought to a close with the knowledge that the security measures were based on an insecure foundation, that being one of unbridled fear, mostly fear of those who "the other."