The question of what death *is* comes before the question of what comes after death.
So far, the second question hasn't come up with my children.
For the first, I think the question came up in the context of road safety or some other health and safety issue (maybe snakes, spiders, peanut allergies, sunscreen, disease, poisons... so many, when you think about it!) we explain why certain rules are important by explaining some things that might happen otherwise. I know that we've mentioned death as a unlikely worst-case scenario at least once, and I think we said something like...
When someone dies, it means we never see them again. They go to sleep and never wake up. They are gone - they can never, ever come back, no matter how much we miss them. If you die, we would be very, very sad, because we could never, ever see you or talk to you or give you hugs and kisses again.
The intent was to convey the seriousness of death. In that context, death is not something to be reduced in magnitude by introducing the concept of afterlife.
When the second question comes up the first time, I don't think it will be time for deep scientific analysis. It will be a time for comfort. Kids need comfort.
Perhaps I'll say that I don't know, and that nobody knows for sure, but that maybe when you die it's like going to sleep forever, or that maybe the person who died can still see and hear us even if we can't see them. Or, perhaps I'll just present the easy fiction of Heaven. It will depend on the circumstances at the time.
Later as they learn to think more deeply, we can examine and criticise various death philosophies.
On a side note...
It's interesting that children (and adults) have no problem with the concept of death for animals and people with which they have no personal relationship. That spider (we just squished) is dead... that cat (by the side of the road) is dead... that man (on the news, or on that TV show, or on that video game) is dead... Benny's dad (who I don't know, but I go to school with Benny) is dead. Children have no problems at all with those ideas. In that non-personal context, dead means dead. Inert. Devoid of all activity. A non-living thing. The life of that thing is
gone.
It's only with people and pets that we have constructed a deep and persistent model of within our selves, those who are part of us, for whom we feel grief. A child cannot let go of the persistence of that person's internal representation... that is why they ask where that person is, even when they know the person is dead. That the person might no longer
be is not a possible conception... it is quite literally
unthinkable. I think that's where the idea of afterlife comes from... it's not the question of our own post-death persistence that troubles us most, its the question of "Where is Mummy?" and (even more heart-wrenching) "When will she come back from Heaven?" (Not that I've had to deal with those questions. And I dearly hope I never do.)
