Congratulations to you all
As a child within the family we would refer to the grandparent as Granny Surname, Grandpa Surname but when I spoke directly to them I would use Granny, Grandpa etc. As it happens everyone else called my Grandpa "Pops." Both my parents died before my children were born, something I've always been very unhappy about, and my mother's sister asked if she could be there Granny. She is referred to as Granny Cardiff as she lives there.
My children called their maternal grandparents Granny and Granddad. There are more possibilities for grandmothers, Nan, Gran, Grandma, NaNa
These days young children call their elders by their Christian name. There is the most charming and engaging young lad, five, who lives on our row whi is very comfortable with calling me Paul and likewise all the other neighbours by their first names. I think it's great.