There's no need to keep your child away from your parent out of fear that one or the other might say or do the wrong thing. Being mentally sick isn't like being physically ill and having to lie in bed. Continue usual interactions so that your child sees that Grandma hasn't suddenly changed or grown infirm, especially in the disease's early stage.
Later in the disease process, it can be upsetting for a child to see a greatly altered grandparent. When your parent no longer recognizes or shows interest in your children, it's not in anyone's interest to force the relationship. At that point, it's best to take your cues from the behavior of each.
