Much of “The Rocking-Horse Winner” focuses on the relationships within a family and how those relationships are shaped by family finances. What does this story seem to be saying about how parental issues with money can impact the entire family system?

In "The Rocking-Horse Winner," the entire family is dominated by the perceived need for more income, so much so that it seems as if even the walls of the house are crying out that the family needs more money.
This sense of not having enough money is clearly not rational. It would be rational if family members were going hungry or living in a crowded, filthy environment. The narrator, however, describes a clean, spacious, comfortable home with a garden, servants, a nursery filled with toys, and what seems to be a prosperous middle class lifestyle. Yet the mother, who suffers from an inner sense of emptiness and an inability to love, can never get enough money or things. She clearly has serious psychological issues that in this day and age she would probably deal with through counseling, but in this story, these unaddressed issues come to have a toxic effect on the family.
The mother's irrational need for more and more things drives the entire family to feel uncomfortable and drives the needy boy, Paul, to try to earn her love by getting more and more money for her. He is too young to understand that no matter how much he gets, it will never be enough. The yearning hunger for money is simply the mother's attempt to mask and compensate for a deeper inner lack, probably a profound sense of worthlessness.
This quest for money literally reconfigures the family, for it leads to Paul's death. This symbolizes how the unresolved issues of one family member, manifested as a disturbed relationship to money and spending, can have a toxic effect on an entire family system.