But what would I say to a child about death?
First I'd agree that it is horrible. There're no two ways on that! Then I'd say that we all grieve for loved ones who die, and in a way, grieve for all who die; loved or not.
Because my children know that the world has turned from fellowship with God and rejected his life, death, which is separation, flows from this; they also know that God intensely loves us even though we (as in all people over all time) have corporately rejected him and so has provided for us to return to fellowship with him.
Those who want to be in company with God will be with him for ever, those who don't won't, and they probably wouldn't want to anyway.