I'm still trying to settle on the best story for how Santa gets in the house without a chimney. I haven't been asked yet (my oldest is barely 3), but it seems to be between (1) he shrinks himself down to the size of an ant and enters under the door; (2) he magically creates his own chimney and fireplace just to enter; or (3) he's got a master key that works in every door and lets himself in.
We have a 'key' that we hang from the door handle on the outside on Christmas Eve. After he uses it he leaves it hanging on the inside of the door.
I think the master key is believable.