Y.C.
Dear H.,
I feel for your situation. I have a 2-year-old son and he's very close to his grandparents. I frequently think about the time when I have to tell him about his grandparents leaving him. Perhaps you already know how to share with your daughter but would like to hear about others' views.
From my side, i'd probably start by sharing with my son the transient nature of life, using flower as an example. How they come to our lives and bring us joy with their beauty and how one day, they will leave us in their physical form but we'll always remember them in our hearts.
Especially if we have taken pictures with them, we can think of them when they are gone by looking at the pictures. Sometimes we'll miss them and tears would come, but after that, we'll remember the good times and there'll be a good feeling in the heart. And we'll be happy that they were in our lives once.
And then you can say that like flowers, people in our lives also come and go. Grandpa is at the end of his life with us now and he may go at any time. We would be sad when he's gone. Tears may come when we are missing him. But then we'll remember him and the good memories together, we'll keep him in our hearts and that would bring us comfort.
And perhaps you can start a photo album with your daugther, putting together photos with your father, since her birth until now, or even with you when you were young. And if your father is able to look at the photos, maybe one day you can share the photos with him together with your daugther. I think he would like that, and both you and your daughter would have a chance to reminisce with him and spend meaningful moments together.
It would also be gratifying for him to hear from you how a good father he has been to you, or those positive moments you remember from the past. And even if he's unconscious, you can still do the same. I believe they can still hear us.
Your children are lucky to have a mum like you! :)
All the best,
Y.