I feel for the parents, but to the one who said they would never forgive them selves, why?? It was nothing they did, like someone else said, it could've happened while playing sports or any number of things. My uncle who has ridden coasters forever, was hospitalized for several weeks due to a heart rhythmn thing, shortly after returning from a family vacation, it happened to be WDW---did my family blame disney? No, thats just crazy, it was a condition he has had his entire life, and noone ever thought or had reason to check it.
I think this type of thing is more common than most people would ever think.
You have to live your life today, no one knows what tomorrow will hold.
Sorry if this offends anyone, just my 2cents.