The house has infantilized the Hadleys by doing everything for them, and doing it better than they could. Their satisfaction is catered to in every way. The parents recognize this, and see the danger in it, because they know what life was like before they had these comforts and they can acknowledge how it's affecting their happiness and their relationship with each other and their children. The children, on the other hand, have apparently never known or don't remember life before the house was there to do everything for them, and so they perceive a life of constant gratification as normal.
Bradbury depicts this as a dangerous thing, but the nature of the danger differs for the parents and the children.
For the parents, they realize that they've begun to feel unnecessary. They have no true role or purpose in this home, especially since they've given away the authority they had over their children and replaced that relationship with machines. This lack of purpose has led to them depending more strongly upon drugs, as well as being less aware of the passage of time; they don't realize how long the nursery has been set to Africa.
For the children, they are already depicted as "excessively" coddled, in the words of McClean, the psychologist, but the true danger lies in the fact that they don't respect their parents nor look to them as true authorities - the children see themselves as the authorities and the technology theirs to command. This would be inconsequential in the immediate term if not for their ability to see their fantasies played out in graphic detail through the nursery - whereas a real child would have to imagine their parent's death, the nursery can do it for them, repeatedly, which appears to have set both the children and the nursery into a psychopathic rut.