This is actually something we talked about at length in my development course.
As others have said, relating death and sleep is actually one of the worst things you can do, as it could make the kids afraid to go to sleep (and it's pretty inaccurate anyway.)
A really good example of talking to kids about death can actually be found in Sesame Street, the episode in which Mr. Hooper dies (there used to be a clip on Youtube; can't find it now.)
They neatly avoid any mention of the cause of his death or any particular religious or anti-religious sentiment.
Big Bird asks all the usual questions little kids ask: "When is he coming back?" "Who's going to take care of me?" "Why does it have to be this way?"
(Answers: "He isn't coming back, he can't." "We'll all take care of you and make sure you're OK" "Just because.")
Granted, I don't think "just because" would've satisfied me as a kid, but it actually relates to an earlier scene.
So I would probably log on and show them that.
If I couldn't, I might use the cocoon analogy: death is like a butterfly coming out of a cocoon. The person's spirit emerges into a beautiful new existence, leaving behind the body, which has nothing else to do and decays.