Sorry, Ellen, I was a little confused as to if this advise was to you for your little children or the adult (and 12yo) children you mentioned.
I think ALL children should be kept informed of g'parent's health in a manner that is reasonable for their age. The teens/preteens are especially vulnerable to misinformation, and they are learning 'good' and appropriate behaviors from the grown-ups in their lives and this includes the grieving process. When you hide things like death and sickness, I think you make it taboo and scary. Death is a part of life and kids need to know this.
For the adult children, I think they are old enough to call and ask for routine updates on g'ma's health, but someone should call when there are major changes (hospitalization, etc.) so they are not in the dark. Post high school age are not 'kids' anymore and should take some responsibility for being citizens in the family!
For what it's worth, I've had the joy of lots of grandparents and great-g'parents and the sadness of losing many of them, too. I am very glad my family included me, even as a child in the process of dealing with their illnesses and death. I don't remember being stressed over it. As I have gotten older, I realize that death and sickness is not some big and shameful mystery and we can get through it together, as a family.