My oldest GD, 6 yrs old calls me Grandma. Our 2 yr. old GD calls me MiMi, her choice. We started out with her calling me grandma, she then went to Grammie, Now to MiMi. The kids usually decide. My husband is PaPa.
My Ex-mother-in-law wanted to be called Granny. My oldest son who is the oldest grandchild ended up calling her Googie. It stuck now everybody calls her that. Even the nieces and nephews.