There are many books for children, even picture books, about dealing with death (of animals, pets, and people) and grief; you might look at how they handle the subject. Death is a real thing that children will have to deal with at some point, and a good book will help them understand (at their level) and process it.
A few to start with:
Nana Upstairs and Nana Downstairs, by Tomie DePaola - A boy learns about death when his great-grandmother passes away.
Ida, Always, by Caron Levis - A polar bear in a city zoo learns that his best friend is dying.
The Tenth Good Thing About Barney, by Judith Viorst - After a boy's pet dies, he tries to think of ten good things to remember about him.
Ocean Meets Sky, the Fan Brothers - After his grandfather passes away, a boy dreams his way to a fantastical place the old man always talked about.
ETA - If you want a chapter-level book that deals with death as part of (but not the whole) plot, try
A Dragon's Guide to the Care and Feeding of Humans, by Laurence Yep and Joanne Ryder; when a dragon's elderly pet human dies, the woman's granddaughter makes a poor substitute - but both have to find ways to deal with their grief.
For a little older kids, Katherine Applegate doesn't pull punches.
The One and Only Ivan deals with death, both in Ivan's memory and in a character in the book. The Endling trilogy is centered on a character who is the last of her species; she sees her parents dead after a slaughter, and carries the grief and anger with her through the book as she tries to find help (and maybe more of her own kind, who were hunted out by humans while no other intelligent species in the land chose to interfere).