The Veldt read this, "It's just that the nursery is different now than it was." Later in the story, when he attempts to change the nursery’s appearance, it doesn’t listen to him because Peter adjusted it so it feels different and so that he is the only one that controls it. This means that Peter and his sister were in control of the lions and made them attack the parents. Therefore, since the children made the lions eat their parents, they are responsible for their death.
Further in the story, you see that Peter and his sister lock the parents in the nursery through these quotes which describes who closed the door, and who was on the other side, “Why, they've locked it from the outside!” and “He heard Peter's voice outside, against the door. "Don't let them switch off the nursery and the house," he was saying.” This shows that the children are responsible for their parents' deaths because they locked their parents in the room where they would die. Therefore, since the children locked their parents in the room, the children are at fault for the death of their parents.
In conclusion, the story shows the children are at fault for their parents because the kids committed the final acts that caused the parents’