Final answer:
The realization that upsets Wendy is the discovery of the realistic virtual reality created by the nursery, leading her to feel inadequate as a parent and want to go home.
Step-by-step explanation:
The realization that upsets Wendy and makes her want to go home is the discovery that the nursery is creating a virtual reality that is indistinguishable from the real world. Wendy and her brother Peter, who had been exploring an African safari setting in the nursery, returned home and shared their experiences with their parents. This revelation causes Wendy to feel inadequate as a parent and unable to compete with the immersive and realistic experiences provided by the automated house. It also leads to George and Lydia Hadley's realization that their children have become too dependent on the technology, jeopardizing their emotional well-being.