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.