Personification—or the attribution of human characteristics to non-human entities—is largely used within Ray Bradbury's short story "The Veldt" in order to provide a sinister tone to the events which take place within the "HappyLife Home" that has been purchased by the Hadley family.
The other educator who responded to this question has done quite a thorough job of outlining instances of personification within the story, but I will elaborate in order to provide more context.
After Lydia and George first encounter the lions voraciously feeding on a dead animal in the the African "veldt" that has been manifested by the nursery, the couple is faced with the problem of dealing with their children's obsession with the room. Lydia has begun to regret purchasing the house, which in meeting her initial desires of relieving her of parental duties has also rendered her useless to her children, as she vocalizes by saying,
That's just it. I feel like I don't belong here. The house is wife and mother now, and nurse for the children.
The personification of the house as having assumed the marital and motherly responsibilities is quite disturbing, and foreshadows the couple's later demise.
After Lydia and George discover the children are disobeying their orders to stay away from the nursery, it is noted that:
Although their beds tried very hard, the two adults couldn't be rocked to sleep for another hour.
Again, we are given a sense of artificial, human-like comfort being provided by the house.
A psychologist who comes to assist George and Lydia decides the couple spoils their children, which has resulted in resentment now that nursery access has been blocked. When observing the room, he states,
No wonder there's hatred here. You can feel it coming out of the sky. Feel that sun.
The house i no longer just acting. It is also feeling and expressing emotions.
The sense that the house is a living thing is once again reaffirmed when Peter begins screaming,
"Don't let them do it!" cried Peter to the ceiling, as if he was talking to the house, the nursery. "Don't let Father kill everything."
The children clearly regard the house as a living, breathing entity, and the suggestion that to power it down would be to "kill it" only makes that concept more alarming. Ultimately, this sense of agency is manifested when the lions projected by the house kill the parents.