Why doesn't Paul divulge his "secret of secrets" about the rocking horse to Bassett or to Uncle Oscar?
The author does not divulge Paul's "secret of secrets" to the reader until near the end of the story. D. H. Lawrence constructed "The Rocking-Horse Winner" with great exactitude. He had to make it credible that a small boy would be able to make a fortune by picking winning horses when Paul had no way of getting to the racetrack or placing his own bets or handling his own money. This explains the author's creation of both Bassett and Uncle Oscar. Bassett handles the money and Uncle Oscar is the one who passes it on to Paul's mother. Paul doesn't tell either man how he comes up with the names of winning horses. That is his secret of secrets.
Paul's secret of secrets was his wooden horse, that which had no name. Since he was emancipated from a nurse and a nursery-governess, he had had his rocking-horse removed to his own bedroom at the top of the house.
So the horse, rather shabby, stood in an arrested prance in the boy's bedroom.
Paul's greatest fear is that someone will make him stop playing the races. He keeps hearing those voices saying, "There must be more money. There must be more money." The only way he could be stopped from betting on the races would be for his mother to find out what a huge sum he was winning. She has already hinted that she may be thinking of terminating his involvement with horse racing.
"Besides, I think you care too much about these races. It's a bad sign. My family has been a gambling family, and you won't know till you grow up how much damage it has done. But it has done damage. I shall have to send Bassett away, and ask Uncle Oscar not to talk racing to you, unless you promise to be reasonable about it: go away to the seaside and forget it. You're all nerves!"
If she fired Bassett it would be a serious blow. How could Paul bet on horses? He would have to bet through his uncle--but his mother shows she is thinking of telling Oscar to stop talking to his nephew about races, and Oscar would have to comply--although Hester does not realize she would be killing the goose that is laying the golden eggs. She doesn't realize that all this nice money that has been coming to her as if by magic is being generated by her own little boy.
Paul keeps his secret of secrets from Bassett and Uncle Oscar because the boy realizes he is doing something that borders on black magic--witchcraft, sorcery, necromancy. Who are the invisible spirits who are giving him the names of future winning horses? And why are they doing it? There is something nearly satanic about it--and the reader feels this when it is finally revealed through Paul's mother's eyes that the boy is riding his rocking-horse in a demoniacal manner in order to get to where the luck is. But there must be something more than luck involved. Paul is afraid to confide in Bassett or Uncle Oscar because he senses intuitively--being a very intuitive child--that the two men might be alarmed and might go so far as to warn Hester that her son is in danger of killing himself, or losing his mind--or losing his soul. Paul knows he is doing something wrong, although he is too young to understand just what is wrong about it. There is something demoniacal about the rocking-horse itself. It has the occult power to carry the boy off into another dimension. Obviously the time could come when Paul might never find his way back.
When both the reader and Paul's mother discover that Paul is riding his wooden horse so madly, it is the beginning of the end. Paul picks his last winner that same night.
"It's Malabar!" he screamed in a powerful, strange voice. "It's Malabar!"
Paul collapsed after that and was confined to bed for several days. After Bassett comes up to his bedroom to tell him that Malabar, a real longshot, won the Derby and paid fourteen-to-one, Paul confides in his mother:
"I never told you, mother, that if I can ride my horse, and get there, then I'm absolutely sure - oh, absolutely! Mother, did I ever tell you? I am lucky!"
"No, you never did," said his mother.
But the boy died in the night.