I don't know if I would have understood how serious my father's unresponsive state was, but I would probably understand that something was off about him, if I had seen him like that. And I would certainly have been very aware of how outside of the norm my mother's sobbing and her demanding I go to the basement would have been, and knowing my reactions, I would likely have been in the background, watching silently, heart pounding, trying to understand, afraid because she was afraid, even if i didn't understand why.
Edit: It would also stress me out if I was kept away at such a time.
I don't know if you're lack of reaction is due to ASD or young age. But if you're feeling guilty over it, that's understandable, it's just a typical human reaction if you do. There is no need to though, kids are very different and pick up things at very different ages. Things will go above kids' heads, that's just the way it is. That's why kids both suffer and keep playing in wartime. Kids get some things, but not the full picture and all the consequences.
They also grieve hard for a short time and play five minutes later. They lack focus for such things, which can seem cold, but is natural because they are kids.