It's rough, eh, Penelope?
Best thing I've come up with is the "red thread" legend - might be Chinese?
So, with apologies to the Chinese for my clumsy rendition - there is an invisible red thread that connects a baby to it's mother's heart. It can never be broken, no matter how far away you are from each other, or how old you are.
When he was unwell last year he "got" that Commander Hadfield was bobbing about in the ISS, but, the red thread between him and his mum was still there, still working, even though he was a grown-up and in space (DS is a bit of a science geek, so, the analogy worked for us)
So, we both wear a red bit of wool round our wrists. It helps him go to school, or when I'm away with work, seems to lessen his panic.
He isn't crippled by this, he's not in an acutely distressed state, but, death is a big thing for wee people. Big for any of us.
We had a chat too about what would happen if I died? Who'd go to the shop, do the cooking, take him to Cubs? So, he kind of worked out that it'd be sad, but, that life goes on and that I'm basically disposable. Which wasn't the conclusion I was hoping for, but, it'll do for now.
I don't have a terrific faith, despite many efforts, but, I did use the Bible's version of Heaven - that the streets are paved with gold, there's no death, disease or suffering and all the people love you are waiting there for you. "So, are there x-boxes?" "Do you think you'd need an x-box?" "yes" "then, I expect Heaven's got x-boxes, the Bible says it's got whatever you need" "cool".
Must go and google what other faiths have to say about The Afterlife.
Hope your daughter settles down, my understanding is that kids get a sense of their own mortality at about 7-10, so, she's bang on track. xx