I wouldn't go out of my way to introduce a child to tragic things, but I have a story to back up that sometimes it's right and necessary for them to know them, and it can be done. DH was very seriously ill (so that for more than a month we thought he might die, and for a few days we thought he almost certainly would) when DS5 was 2.5. As long as his dying was just a small possibility, I didn't press it on DS, although I did carefully avoid saying things like "the doctors will make Daddy better and he'll come home" and ask the nursery staff to do the same. The day I fetched DS from nursery to take him to ITU and see DH sedated and on a ventilator, I did talk to him on the way about what he'd see, and I did say "Daddy may die" and try to explain what that meant. Not a conversation I'll forget in a hurry. But there was a real possibility that we'd get to the hospital and find him already dead; and obviously, I wasn't in a state where I could conceal that something was very, very wrong.
Fortunately DH did recover! (Although he never was diagnosed- that in itself has shaken all our worldviews somewhat; I knew there are things we can't cure, but that there are life-threatening illnesses we can't even diagnose, even with the full panoply of testing and experts available today...) DS has very much done the spiral thing: he's had several phases of wanting to talk about that time repeatedly. I don't think he actually remembers visiting ITU. I don't regret talking honestly to him at the crisis time, at all. At the same thing there are things we shield him from - e.g., DH still has many things about his physiology outside the normal range and will be followed up for the foreseeable future, but given that he's well, we tend just to say he's having check-ups, and not talk details in front of DS.
