This post seems to be gone, even though I wrote and autoposted it to go up yesterday, I thought. And now I can't even find the original question. So here's a paraphrase of the question, and a reconstruction of my original answer:
"My mother has cancer that's spread to her liver, and is undergoing an extremely aggressive chemo protocol that will go on for basically the rest of her life. Because of this, her immune system is compromised and she's in danger of getting sick and dying of things that the rest of us shake off easily.
How do we deal with this? My three-year-old loves his grandma, and doesn't understand why he can't see Nonnie anymore. And she misses him. Is there anything we can do, or are they never going to be able to see each other again? That would break my heart, as well as both of theirs."
Oh, this is breaking *my* heart. I'm so sorry about your mother.
I don't know if there is any way your son can see his grandmother. There must be a patient coordinator or social worker who can investigate this for you, to find out if there are ways your son can see your mom without endangering her life.
In the meantime, you can use a lot of the ways people have suggested for grandparents to keep in touch with grandchildren when they're living far away. There are many suggestions here, and most of them consist of talking by phone or videophone or Skype (free!), video or audiorecording your mom reading books to your son so you can play the files for him, and recording your son talking to her so she can see/hear him.
Has anyone been in this situation? I'm guessing some of us may have been unable to see someone for a short period of time, but this is a permanent situation for them, it sounds like. Any ideas how to help them cope?