What is the significance of the expression “There must be more money!” to the short story? Namely, how does the expression specifically relate and/or develop a literary device – plot, conflict, characters, mood, tone, theme, etc

Much of the narrative relies on magic realism to illustrate the disease of greed. We can take the whispers as a metaphor of mother's greed or we can take them as something more literal. I either case it is greed unchecked that feeds on itself manifesting the longing that is personified by the walls. This incessant stress of always wanting more can be felt by all the children.