I don't know what to say a kid at all about this topic. I think it depends maybe more on the personality of the child to begin with how they feel about death. I'm in my 20s and death still fucking scares me even though I'm aware of some of the philosophical arguments (such as death will happen no matter what, you didn't exist before you were born, etc) of why the fear of death is irrational and even though they are logical, I just can't seem to stop my emotional fear of it.
I think my consciouses will cease to exist which bothers the hell out of me but to me what is likely true is more important than what makes me feel good. I'm not one of those materialist that are fine with the fact of death. However, some people believe what I think is likely true and seem to live with minimal fear of death.
Some people actually hate the idea of heaven, (not even talking about hell) believe it or not. The idea of someplace were you have to be happy like some kind of illegal drug substance influencing your feelings for eternity celebrating some deity bothers some people.
Think of what the child's personality is like , and what will bother that particular child the least about death I think may be the best recommendation. I think my consciouses will cease to exist which bothers the hell out of me but to me what is likely true is more important than what makes me feel good.