My MIL died of cancer shortly before this past Thanksgiving. My own mother died a year ago this past October. My children are currently 22, 20, and 17. They had a very close relationship with my mother and a fairly close relationship with my MIL. Both women died of incurable cancer. My mother chose not to undergo a second round of chemotherapy, knowing her time would be limited. My MIL was prepared to fight until the end, although her doctors would not approve any further chemo treatments for her (about a month before she died.)
We made a point of telling our children that their grandmothers cancers were incurable, though treatable, from the time we learned it ourselves. We also told them when (aggressive) treatment was discontinued. This way our children were prepared for the news that death was imminent when that time came. We asked for the grandchildren to pray for their grandmothers; it’s important for all of us to have something we can DO about the coming death of a loved one. When death was but a few hours away we made a point of bringing my middle daughter home from college so she’d have a chance to say, “Good bye.” (In the case of my mother we don’t know if she was conscious or not.)
Yes, it was hard for my children. Their ages are a bit older than yours but I know it was important to THEM that they could help their grandmothers by being with them. And they knew they had done all they could. They got a chance to say their good byes.
You don’t know how much time your father REALLY has. It sounds as if your father’s terminal condition has perhaps taken the family by surprise even though you knew he had cancer. You may still be trying to process this yourself. But please let your children have as much of that time as they can. If Grandpa is going on hospice then let them know what that means. If he’s continuing with some treatment to maintain health as long as possible then explain that he’s not fighting the cancer anymore; he’s just trying to have the best quality of life possible, then explain that. If he’s still trying chemo, radiation, or surgery (which I doubt based on the time frame given) then explain that he’s trying desperate measures.
You don’t need to say, “Grandpa has only three months to live,” because you don’t actually know how much time he has. But be clear to your children that this holiday season is almost surely Grandpa’s last so you want to make the best of it with him.