You may have already seen these previous threads on talking about death...
1) Questions about death - normal?
2) Death: Reassuring very young child
3) Existential Depression in a 5 year old??
4) Gifted kids dealing with death.
5) Preparing DD for expected death in the family?
6) Fearing death - asking big questions
7) Discussing death with a 3-year-old?

In addition to the brief roundup above, there are some interesting resources on Hoagies' Gifted Education Page:
1) Hoagies' Gifted Education Page offers a collection of resources on Grief & Mourning.
2) Hoagies' Gifted Education Page hosted a September 2017 Blog Hop on a related topic: Spiritual/Philosophical Anxiety.