Reading to a child periodically (I like to say once or twice a month) about a topic having to do with death, helps them process a death that has happened in the not so recent past, as well as for the future. By reading books with them, it gives you and them the opportunity to ask questions about death and also answer questions about loved ones who have died.
Take advantage of the death of a goldfish to really talk about death. Have a burial and sing a song to the fish. One simple song I always like to sing with the preschoolers is "Goodbye, NAME OF ANIMAL, Goodbye" or "Adíos, NAME OF ANIMAL, Adíos". Then I ask if anyone would like to say something about the animal that died. Then we sing the song again and continue this process until everyone who wants to say something about the pet does so. We try to bury them in our garden where they can help our vegetables flourish and we thank the fish for that.
National Institutes of Health references the developmental stage of a preschooler "preschool children usually see death as reversible, temporary, and impersonal. Watching cartoon characters on television miraculously recover after being crushed or blown apart tends to reinforce this idea." We can help them cope with the finality of death by talking about it openly, honestly, and respectfully.
Of course, it's okay not to have answers and to share your personal beliefs about what happens when someone dies. There are many great books on this topic that are non-religious and some that answer questions in a more spiritual way. Here are a list of some of my favorites. I believe it's a good idea to have two to three of these on your bookshelf at all times so that your child has the opportunity to choose these books and you have the chance to read one every so often.
Saying Goodbye to Lulu by Corinne Demas
The Tenth Good Thing About Barney by Judith Viorst
Nana Upstairs and Nana Downstairs by Tomie dePaola
Badger's Parting Gifts by Susan Varley
Tough Boris by Mex Fox