The keys

*missed this, not kissed this :-)

Because the keys were never real. It was all happening in Malcolm's head, after all. Even from the beginning, the keys didn't correspond to the room the person was staying in (Carolyn Suzanne, for instance, wasn't staying in 10 even though the 10 key was found with her head.) If you want a more practical explanation, the killer left the keys with the victims, counting down from 10, so the Yorks would each be given their own key after death, in the order they were murdered. The 10 key was left with the first victim. The 9 key with the second. And so forth. The dad was the fourth of the group to die, counting down from 10, so the killer left the 7 key with him. The mom was the fifth to die, so the killer left the 6 key. They had nothing to do with the actual accommodations given to each person. Presumably they also had the actual key to their rooms (which would have been shared by George and Alice) but those keys play no part in the subsequent events. If you're really determined to interpret it in real-world terms...remember when Larry was asked about the keys he had, and he said there were two sets for each room, plus his master key? So every room, even the ones being used, also had a second key hanging in the main office, so the boy could just grab those from the wall and leave them with his victims.