The plot in "The Rocking-Horse Winner" is generated by the mother's greed and her constant harping on the need for more money.
This leads to other family members feeling inadequate, and to them sensing and obsessing about the constant need for more money.
This leads to the boy riding the rocking-horse, betting on the races, etc.
The theme of the story is revealed and emphasized by the mother's greed and the constant, haunting awareness that the family must have more money. Of course, the tragic conclusion demonstrates and solidifies the theme, as well.