Maria Popova of Brain Pickings did a beautiful review of children's books that address the topic and Mark Levine of The New York Times Book Review wrote on four children's books that take on death and mourning.
When I was teaching elementary school, I generally chose lighthearted books with a customary happy ending for read aloud. Many children's books do touch on loss, but in a distant and blink-and-you'll-miss-it sort of way. Some of the classics address transformation and quite profound loss in a really touching way, if you know how to use them to prompt an honest conversation. I often didn't. But my struggle most frequently reflected my own discomfort, not my students'. I recognize that they really wanted to talk about meaningful subjects, and many of them seemed to naturally recognize that death and loss is incredibly meaningful, and has something very important to teach us. Many of them had experienced or were experiencing very challenging losses, with little support (I was at a very poorly resourced public school - a whole separate challenge topic!) and a short supply of books or adults willing to dive into it with them. How could we do much, much better? What are you seeing that is inspiring you?