To our oldest grandchild, we are Sue and John. Our daughter lived at home and when the GS started to call us Mom and Dad like she and our sons called us. We corrected and said no, this is Mom, we are Sue and John. It stuck. To this day he still calls us by our first names. He, however, did teach siblings, that we are GrandmaSue and GrandpaJohn.
Funny story: Our DD was in nursing school and our GS was w/the daycare mother. He related that he did not have grandparents. When DD returned to pick him up, the daycare mother asked what had happened to her "dead" parents. Our GS got really excited when she explained that we were much alive and he knew us by our first names.