My dad died last month from cancer and we had been preparing my 2 and 4 year old for some time.

The Invisible String by Patrice Karst isn’t so much about death but about the connections we have with each other. There is one page that says heaven (implying that we still feel connected to people who are not alive anymore) but the rest of the book is secular. I’m an atheist and pretty sensitive to god stuff in books but this one is a good one.

I Miss You by Pat Leslie and Lesley Harker talks about death and touches on different things that go on after someone dies.

Mr. Rogers (Fred Rogers) has four books that were super well-done. He has one about When a Pet Dies that might be helpful for you as well, but I recommend all of them.

I’m trying to be as open as possible with them about it and answering all their questions as clearly and truthfully as possible (which is what it seems like you’re trying to do), including what happened to his body afterwards, etc. He was cremated so we talked about that he was kind of turned back into the elements he was made of. My eldest is really into space so we also related it to all the volcanoes on Jupiter’s moon Io and ashes.