“I was privileged to have all four of my grandparents into adulthood (in fact, all were at my wedding). As a result, my grandparents were “Grandpa X” and “Grandma Y” (with X and Y being their first names).”
My kids b”h also have all their grandparents so they call them “grandma X” and “grandma Y” with X&Y being their LAST name.