I am so very sorry for your coming loss.
My 4 year old grandson (whom I'm raising) was a part of our in-home hospice for my Dad when he was not yet 3, and for my Mom at 3.5. Before hospice began for Dad I told him that he was very sick, once hospice began I told him that Tata would be going to heaven soon (my belief.)
Dad was on hospice 26 days. His kidneys began shutting down a week before he passed, the visiting nurse saw all the signs he'd be going soon on a Friday, she said it could be 1,2 or 4 days, she couldn't say. It was 7. No one can say for sure how much longer a person has. That night Dad had a great time with family and friends, alert and happy, our final gift. After that he slept more and more until he went into a coma on Tuesday. I still took my grandson in to his Tata to kiss and hug him, I wanted Dad to know he loved him, and people DO still sense this even in a coma. On Wednesday afternoon he reached for my Mom's hand when she went in and sat next to his bed. The hospice chaplain was visiting and it brought him to tears, he was touched, he said that's how he knows people are still hearing and sensing even while comatose. That night, in a coma, he raised his arms in worship while my brother played his guitar and sang worship songs, but didn't communicate with us.
Throughout this time I kept reminding my guy that Dad was leaving soon, when Dad passed Friday night I woke my grandson and took him in to say goodbye, and he kissed him and hugged him and told him he loved him. He saw when the mortuary picked Tata up, I wanted to have a reference point for him when he questioned where he was later.
With Mom I "knew" months before she passed, she lost the will to live. So, I started preparing him a bit sooner. When he asked why she slept so much I told him she was tired and ill, her kidney disease progressed quickly, so fast her doctors were surprised. We sensed she might not be with us long so had an early Thanksgiving in September. It turned out we were right, she passed Nov 10th. She was happy to have her family all around. She started saying goodbyes a couple of weeks before she passed, she was ready even though we weren't. We called to start hospice on a Monday, she started Tuesday night, she went onto a coma Thursday evening, passed Friday night. The same, my guy said goodbye, he kissed her and hugged her. He went to both memorials and funerals.
Sorry this has been so long, my point is, my Dad wasn't ready, my Mom was, I was honest with my guy that they were leaving soon. It helped him to process it all. I'd advise you tell your daughter the truth, as many times as necessary, it may take awhile for her to understand. My grandson just asked me yesterday how long Nana and Tata will be in heaven, I told him forever. He said, "Awwwww, I miss them." And it's been a year and 4 months since we lost Dad, 7 months since we lost Mom. So it takes a while for them to truly understand. Hard as it is, tell your daughter her grandfather is very, very sick and will be dying soon. It's fine if she sees you're sad or crying, grief is an emotion we all have, it's normal.
And I would try to go see your Dad now, to say goodbye, it doesn't matter if he can't respond. He may be holding on, waiting for you. Both my parents let go when they'd finished with their goodbyes.
{{Hugs}}, I know this is hard.