Hi op I am so sorry for what you are going through,
We received the same news with my aunt, dads sister, 3 months, they were devestated,
my mam rang each of us in turn to "remind us" that she knew we knew we had to make a special effort for the weeks that were left to visit and keep her sirits up, to bring our dc as aunt was childless and she loved our dc as if they were her dgc,
My aunt actually lived for just over 12 months after first discovering she was sick, she went on a holiday with all her siblings, and lived for our visits, the stories of the mischief the smalls got in, seeing new born babies thrive, a wedding and exam results, She lived in a nursing home for the 12 months as the surgery had.left her paralysed,
I would speak to your children again and tel them this is not about their dad or even about christmas, but as they are older and adult enough to appreciate, that every day from hear on is important and that you know they will want to spend as much time as they can with their nana every weekend and free time while she is well enough to enjoy it and draw comfort and strength from their visits,
I'm afraid there will be more weekend to come, and your dc need to realise that family comes before a schedule, even if your ex is too .... To realise that,