How you tell small children what death is, when it occurs, depends very largely on what you as a family believe.
If you believe that this life is all there is and that one day it simply stops, then that is what, if you are to be truthful, is what you have to say. And naturally, you will be truthful.
If you believe in a religion that teaches of an after-life, then the task is, to me at least, slightly easier: Granny is ill, old and tired, and sadly the doctors are not able to cure her. So God has taken her to Heaven to be with him, and there she will be well.
At six, a child will be too young, as like as not, to comprehend if you start offering alternative points of view, along the lines of some people think we just fall asleep and never wake up again, others that we go to Heaven - this sort of thing can only confuse a little one.
Whatever you do, don't use euphemisms, like "gone away" the logical child will ask "When will she come back?" or "where to?"
I have found it important that the adults in the family are in agreement as to what to say, and stick to it, and don't let themselves be caught out saying different or contradicting things.
What you say, and how depends very much on the child you are talking to.
I am sorry you are faced with this difficult sad task.