Who killed the parents in "The Veldt"?

Peter and Wendy killed their parents, George and Lydia, when they locked them in the nursery with the lions in the African veldt hologram that has mysteriously become real instead of just the imaginings of the children.  George and Lydia are increasingly unnerved by their children’s obsession with playing in the veldt.  It is a violent place where the children see lions stalk and kill their prey.  George and Lydia, along with the family psychologist, Dr. McClean, feel that the children are too caught up in the death the place represents.  George and Lydia threaten to lock the nursery and turn off the rest of the technologically-rich house, and the children seek their revenge by luring them into the nursery and locking the door.  The lions take care of the rest.