The situational irony at the heart of “The Veldt” is the fact that the expensive high-tech Happylife Home George and Lydia Hadley purchased in order to make life better for themselves and their children ends up being the very thing that leads to their deaths. Not only did it turn their children Wendy and Peter against them, but it is also responsible for creating the not-so-virtual reality lions that kill them.
Because the house attends to all of Wendy and Peter's needs—feeding them, clothing them, and providing them with endless entertainment in the virtual reality nursery—they stop feeling connected to their parents. Thus, when their parents threaten to turn off the technology throughout the home, the children side with the technology, trapping their parents in the nursery and leaving them to be mauled to death by lions.
The situational irony comes across in the following passage, as the children beg for one last play session in the nursery before their parents turn it off:
Wendy was still crying and Peter joined her again. “Just a moment, just one moment, just another moment of nursery,” they wailed.
“Oh, George,” said the wife, “it can’t hurt.”
“All right—all right, if they’ll only just shut up. One minute, mind you, and then off forever.”
Lydia’s response to the children’s whining—“Oh, George […] it can’t hurt”—is ironic given the fact that it very much can hurt for the children to access the nursery, as seen in the fact that she and her husband are killed in the room soon after.