Horrid situation. 
Excellent advice above. The only thing I'd add is something that worked for us to explain death as a general concept. I was keen to get this into the dc at the earliest opportunity, it sounds a bit morbid but having lost a parent unexpectedly at 5 I knew the cost of delaying these conversations.
DS1 saw a dead pigeon, I encouraged him to look, explained lots about the physical differences, and that it didn't feel pain but couldn't be fixed. Talked about the pigeon family's feelings and the likelihood of pigeon heaven existing. Wondered about the practicalities of pigeon funerals and graveyards. Went back to the subject of the 'poor pigeon' several times over the next couple of months.
I was concerned I'd overdone things tbh, but have always feared dying before my dc understood the concept. Sadly, my b-i-l died soon afterwards and it was much easier to explain 'like we talked about' than to start at the beginning during a time of crisis.
This is just my experience, it in no way compares to your situation but might come in handy for someone.