When I was a child we had two sets of grandparents and they were addressed as Grandma or Grandpa followed by the surname, e.g. Grandma Smith. My children only had one set of grandparents so Grandma and Grandpa didn't need further identification. When my youngest sister had children with two sets of grandparents she made a change which appeals to me - her children address their grandparents as Grandma or Grandpa followed by their given name, e.g. Grandma Donna or Grandpa Carl. Things have gotten more complicated with blended families, step children, half siblings and so on. Case in point, one of my sons married a girl with two daughters and her parents were divorced with her father having a second wife. That son's stepdaughters have 2 biological grandmothers and 3 step grandmothers. One is grandma, another is nan, the third is gran, the fourth is Joan and I'm Nana Kay. It seems to work.