I can still remember my utter terror at realising some day my mother would die. I would have been about four.
It was entirely about being left alone, that those I needed wouldn't be there, that I'd somehow be on my own and I couldn't see how that would be possible and so it was very very scary. Its a big shock to realise that the security of your parents that you have assumed is complete, 100%, rock solid, never going to fail... actually might.
My mother told me she wouldn't die for a very very very long time and if we wanted we could have her stuffed like our stuffed Otter so she'd still be there, which i found pretty amusing but other kids might not.
We didn't get any heaven business as she was fiercely (nay, offensively) atheist but she did explain (probably multiple times over the years as I doubt I took this in straight away first time round)...
Everyone we know and love, who we talk with, spend time with, shares some part of themselves with us. That then shapes who we are, in effect a bit of them becomes part of us. So if they die... the bit of them that is part of us doesn't die, it carries on.
I think she came up with that as the first deaths I experienced were not of blood relatives but of our cat, and then two family friends who had been honorary uncle types, and so that one works better than trying to explain a blood link or umbilical link.
Ultimately I needed comfort and cuddles and time, I don't really think the words matter all that much, but some sort of reassurance does, and it shouldn't be a blatant lie or that will cause problems later on.