Could you possibly persuade the child's father to ease up a little on his no talk about Heaven policy?
Whether you believe in an afterlife or not, the concept exists to make it easier to accept death.
The important thing is to tackle the little one's fear. Has anyone you know died recently, or has she just reached the stage of development where the reality of death dawns? Or has she been hearing too much about the war in the Ukraine?
Saying no-one really knows what happpens when we die is unlikely to make her less afraid, as after all, most of us are scared of the unknown.
I would suggest telling her that for some reason we do not really understand all life comes to an end. What happens when it does, we do not really know, but it could be the start of a great new adventure and that the people and things we love never leave us as long as we remember and love them.
Being told that death is the next great adventure made no real sense to the eleven year old Harry Potter, and will make less to your five year old granddaughter, but it might just be the most useful way of handling her fears.
As adults we know that most people are old when they die, but that young people can die too, but most children worry less if you simply say that neither she nor anyone else in the family is likely to die soon.
Obviously, you cannot say this, if some member of the family is already seriously ill, but that doesn't sound like the case.
If it is the war in Ukraine that is the reason for her fears, you need to assure her that there is no real likelihood of the war spreading to us.