Does the house really whisper and why only the children and the toy hear thr whisper?
In "The Rocking-Horse Winner," the house is personified at the very beginning of the story as having human feelings "as they [the family] felt always an anxiety in the house" (paragraph 3). The house's anxiety grows larger and larger "and so the house came to be haunted by the unspoken phrase: There must be more money!" (paragraph 5). Just as the house is personified as having anxiety, Lawrence also uses personification in making the house speak. Interestingly enough, it only speaks to the children and the toys as both parents appear unconcerned as to the price their children are paying for their greed. The parent's unhappy, materialistic life has pervaded the entire house, and the children feel this misery intently.
As the story progresses, when Paul gives his mother the entire five thousand pounds, the house begins to scream and is described as "mad" when Paul's mother spends the money as quickly as she can. This personification and Paul's terror at the house's reaction foreshadow his death at the end. Therefore, since personification is a literary device that can give human characteristics to nonhuman subjects, the house can whisper.
The story is written in a fairy tale or fable style. Just look at the first paragraph where we are introduced to the characters and conflict. If you view it as a modern day fairy tale, which magical elements, then the house could certainly whisper to the kids.
One reason the children are only able to hear it is because the mother is oblivious to their needs and the house's needs. She is living beyond her means, so there is always a need for money. However, she does little about it. When Paul is able to earn money for her, she doesn't save it. Instead she burns through it, causing the house to call louder than ever for more money.
The fact, that her mother cannot hear the whispers is one reason Paul is driven to ride the rocking horse in order to compensate for the money his mother desires and the money his father isn't luck enough to bring in.