It would be wonderful for your children to have a letter from their grandmother. A simple, honest expression of her values, her reflections on her life, and her hopes for
her grandchildren would be a gift they'd always treasure. But if you suggested the idea to your mother and she refused, there's probably little point in trying to persuade her. She has the right to deal with her death -- and her legacy -- in her own way.
Still, there may be other ways that you can create sustaining memories for your children and help your mother prepare for her death at the same time. Most elderly people feel a powerful need to understand what their lives have meant and what they will leave behind after they die. Your mother may not be able to articulate it, but it's likely that as her death approaches she's trying to come to grips with her own legacy.
You and your family can help her do this. In a quiet moment -- when you won't be interrupted -- approach your mother again. If she finds the idea of writing a letter to her grandchildren uncomfortable, are there other forms of expression with which she feels more at ease? You could suggest, for example, that she talk about her life with a tape recorder on. You -- or one of your children, depending on their age -- could ask her questions to make the process more relaxed and familiar. If she enjoys drawing, she may want to sketch some pictures.
You could also offer to create a photo album of your mother's life. Help her sort through old photos and ask her questions about the people and places, then create captions to accompany the photos. Depending on her degree of illness and the ages of your children, this could be a project they might be able to participate in.
Remember that family time is likely to be one of the things your mother values most, and it will be a source of rich memories for your children in years to come. If your mother's health permits, consider taking a family trip to a place that holds special memories for her. If she's too weak to travel, a short drive and a picnic with her grandchildren may be all that she can handle. If she's confined to a wheelchair or to bed, quiet visits will mean a lot. Bring your camera for these family occasions, but be sure to honor your mother's wishes if she doesn't want to be photographed.