This is a great question as these two excellent short stories offer many parallels, in regard to the way that the actions and preoccupations of parents impact their children negatively. If we consider these two stories from this angle, we can see that it is the dominance of one characteristic (fear and greed respectively) that results in the parents actually gaining the death of their children as thanks for their excess.
Let us consider how this operates in "Once Upon a Time." What begins ostensibly as a fairy tale quickly becomes something much more darker and more shocking. Even though the mother and the father were "living happily ever after," it is clear that the mother in particular is ruled by fear, which results in their ever more frenzied attempts to protect their house. As a result of this fear, their house seems gradually to become more and more like a fortress than a place to live:
So from every window and door in the house where they were living happily ever after they now saw the trees and sky throguh bars, and when the little boy's pet cat tried to climb in by the fanlight to keep him company in his little bed at night, as it customarily had done, it set off the alarm keening through the house.
It is of course this cycle of fear that causes the parents to install the infamous dragon's teeth, that results in the mauled carcass of their child having to be hacked out from the iron teeth. Their fear has resulted in the death of their most precious possesion.
In the same way, we can see how the mother's greed in "The Rocking-Horse Winner" actually resulted in her son's death, as he was driven to ever-greater excesses on his rocking horse to try and win more money for his mother, to whom even large amounts never seem to be enough to satisfy her thirst for wealth and her greed. Note the way that this is explicitly refered to in the final paragraph of the tale:
And even as he lay dead, his mother heard her brother's voice saying to her: "My God, Hester, you're eighty-odd thousand to the good, and a poor devil of a son to the bad. But, poor devil, poor devil, he's best gone out of a life where he rides his rocking horse to find a winner."
Uncle Oscar is able to pick up on the way that his sister's greed and the lack of love expressed towards her children caused Paul, in an effort to gain his mother's love, to surrender himself to supernatural forces so that he could gain information that would win money for his mother.