In D.H. Lawrence's "The Rocking-Horse Winner," the moral one sees depends on the experiences each reader brings to his/her reading.
For me, the moral of the story is that people are more important than things. This is something the boy's mother loses sight of in her hunger for more money to buy more things.
We learn that the mother has "...all the advantages..." In the very first line of the story, we learn she not only has enough, but all she could want: she has "all the advantages." The author also says, "...yet she had no luck." She speaks of luck to her son. Someone unable to have children might heatedly argue that she is very lucky to have children. She is also very lucky to have "all the advantages." I feel she brings a sense of bad luck upon herself and her family by sharing these ideas with others. We know that her son takes this idea very much to heart.
And she demands more and needs more, the boy picks up on the message and the anxiety that rules his mother, and embraces these very adult concepts to his young mind and heart. No child should have to worry about these things.
"Oh!" said the boy. "then what is luck, mother?"
"It's what causes you to have money. If you're lucky you have money. That's why it's better to be born lucky than rich. If you're rich, you may lose your money. But if you're lucky, you will always get more money."
"Oh! Will you? And is father not lucky?"
"Very unlucky, I should say," she said bitterly.
In this short exchange, the boy doesn't learn about the importance of family or love, but that money is almost everything. Luck is everything, but only in as much as it brings in more money. This serious young boy listens to worries of an adult. This mother should not be worrying about having, but should be protecting and comforting her son so that he has nothing to worry about. Life will take care of that soon enough.
When he says he is lucky, the boy's mother shows that she clearly does not believe him. This lack of trust is also unhealthy for a child. Her response is not just disbelieving, but she is bitter in telling him.
The boy becomes saddled (pardon the pun) with the notion that he must prove to his mother that he is lucky—and he can do this by winning races. He is "supernaturally" able to predict winners at the horse races. He makes a lot of money with the help of Bassett. And he secretly shows his uncle what he is able to do at the races. The boy makes scads of money and gives part of it to his mother.
She spends it all; still she is unsatisfied. The boy hears the house speaking, echoing his mother's obsession:
There must be more money! There must be more money!
The boy's mother is greedy; she'll never be happy, so he can never truly be a winner—she is impossible to please. If she was a good mother, she would have made sure to spend time with her children and put her concerns away where they could not be seen or heard by her little ones. She does not appreciate the beauty and innocence of her kids. She turns her son into a small man-child, who while still a kid at heart, has the worries of an adult. Ultimately, this makes him sick, and he dies.
The boy's uncle chastises his sister—the boy's mother:
...poor devil, poor devil, he's best gone out of a life where he rides his rocking horse to find a winner.
The uncle says this because to his mother, the boy was never a winner for who he was. All the things and money she had were nothing compared to the life and well-being of her son.