Pap has Diabetes

Explain that to a child.  My dad has a long history of Diabetes and not taking care of himself.  He has neuropathy in his feet.  He takes a few different types of insulin shots.  He also continues to eat candy and junk food as he has his entire life.  He's 68 now, and he feels old.  We've all asked him to take better care of himself, but he still does what he wants.  So, now we've gotten the grandkids in on it.  When they see Pap eating candy we've told them to tell him they don't want him to die.  Do you think its going too far to tell the kids that candy could kill him?  Or is there something else we should do?