Explaining who god is to a small child:
"Remember when you used to be scared of going over bridges because you thought there might be trolls under them? And then, you found out that 'Billy Goats Gruff' is just a story, and that trolls aren't real?
Well, there's a story about something called 'god'. Some people think it's real, although it isn't. It's just a story; god isn't real.
But, you remember about always trying to be kind to other people? Those people who think the god story is real, they think it's rude to say things about god. So, although the story isn't real, we should still be kind to them and not say things about god, even though we know god isn't real."
That also works for related things like explaining why some people wear sacks on their head and so on.
And it gives a nice introduction to telling children the 'god' story, independently of superstitious import. My own children used particularly to enjoy the plagues of Egypt; 'Boils, aargh! Not the boils! ... Frogs! EEk! -- All useful stuff for general cultural background, just like Billy Goats Gruff, in a way.
(It will be plain I don't think we should take religion seriously. I do think we should try to be kind to people, though; that's something we all need to take very seriously indeed, and teach our children.)