I believe that the parents, George and Lydia, are responsible for their own deaths. The kids were spending so much time in the nursery that it seemed like it was home for them. Than they started creating things like Africa and lions that we believe killed George and Lydia Hadley.
Early in the story the parents had the therapist over and he stated,"I sensed only that you had spoiled your children more than most. And now you're letting them down in some way.” This shows that George has let the children get too attached to the nursery and technology so when he takes something away, the roomed showed death.
Later on in the story, George tried to turn the house of and this is how the kids reacted"Don't let them do it!" wailed