or perhaps there are loads of them out there? A friend has decided her daughter will call her grandparents (and there are 4, to be fair) Grandma Mummy (for the Mum's mum), Grandma Daddy (for the Dad's Mum) and Grandma Firstname and Grandma Firstname for two "step" Grannies.
I would hate to be called Grandma Daddy by any grandchild of mine. What is wrong with just using other names like Nan or Nana or whatever?