Well.... this may sound phony, but it's not. I assume every single day I'm going to die at some point during that day, in my case, probably a heart attack. It's more than possible in my case, so it's not some drama queen thing. I live every fucking day as if that day mattered, I try to make people I care about happy and tell them how much I care. Other than that, fuck it man, don't do things you wouldn't want on your conscience when you die, do what you can to enjoy life, and don't take things for granted.
In this hypothetical case... I'd tell my wife, not my kids or anyone else. Why burden them with something like that, they'll find out soon enough, and what if you tell them and then you don't die? Lots of worry over nothing then. People might imagine it would be better to warn everyone, but is it really? Isn't it more digging for sympathy, or spreading the misery? If it was true you'd probably end up in the hospital eventually, and then they'd all know.
What would I do? Considering my modest means, not much different to be sure. I would try to see my friends a bit more, if I could magically afford it, I'd quit working and see my family and friends all the time. I'd play some games, eat some good food, maybe go camping. I'm a simple guy who's been around a large part of the world a few times, so I wouldn't feel the need to "Go" anywhere.
Would I question my existence? Not at all, worked that out long ago, and I'm comfortable with my actions on this here world of hell.
Now if my wife was diagnosed with something like that, it'd be different, for sure.