DD is very sensitive and anxious about most things, including death.
Lots of "But I don't want to die!" And "I don't want you to die!"
I tell her that dead people don't mind being dead. That when you're dead you don't feel upset or unhappy about it. So she won't mind being dead.
I tell her that I won't die until I am an old lady and she is all grown up. By then the thought of me dying won't bother her so much, and that usually people are very unwell when they die so it's a kind of relief that they stop hurting.
Actually, I hate saying that. She knows that sometimes mummies die when children are little, and I have to say that that was very unusual, the mummies were VERY "unlucky". Death in itself is easier to deal with than untimely premature death.
Maybe that sounds too brutally honest, but DD is 5 & hasn't anxiously asked in a while, so I assume she's satisfied with what I've said, for now.
DS is 7 and has virtually NEVER asked about death, just doesn't worry him.