What sort of parent deliberately scares a child? That is not a game, that is child abuse. Do your daughter and son-in-law have some sort of brain deficiency? Maybe they shouldn't be allowed children, they sound too immature for children.
Please accept my apologies for this comment. It was intended for another question. I don't know how I managed to get it on your question instead. Sorry.
Dear Mary,
In a fairly casual manner, mention that your surgery is a pretty simple matter (not that your daughter will really understand this), and talk with her about the expected recuperation and when she can expect to see you. You needn't get into the specific matter of people not knowing about the timing of death, etc. Hers is a natural fear for a young child.