Backing up bibliolept.Grandparents get called what the grandkids call them.
Bigger granddaughter, 8 years old, calls me poppy, pops or pop. MrsP is granny.
Smaller granddaughter, 16 months old, calls me bappie just now. MrsP is nannie.
I have to say, though, that it would be unusual for a grandmother to be called mama.
Mothers have reserved that particular combination, and variants of it, for themselves.