With magic of course! This reminds me of a conversation I had with my 4 year old brother and nephew recently:
Nephew: I don't have a chimney... so how will Father Christmas bring me my presents?
Me: Oh... Err... I'm not sure! Maybe he'll come in the window...
4 year old brother: Noooooooooo Don't be SILLY! Father Christmas is magickt didn't you know?! He MAGICS a chimney to come down!
We actually have a "magic" key on a big, red ribbon that we hang outside the front door. It's how Santa gets in to leave the presents. Then, he eats the cookies, drinks the milk...and leaves the key sitting on the empty plate. Problem solved!