Why not let the child choose?
My oldest son was first grandchild on both sides of the family. My Mum wanted to be Granny, but he decided to call her Mama. I was Mummy. He called his dad's mum Grandma Roberts (yes always with her last name).
He called my dad Par-par. There was no other grandpa when he was born. Later when Grandma Roberts remarried, he called her husband Grandpa David.
I had a Granny and Great Granny on Mum's side, plus a Grandpa until I was 8. My dad's Mum was called Mar-mar. I named her that when I started to talk and it stuck until she died when I was 16. All the family called her Mar-mar, including my friends.
My younger two kids have Nana (xh mum) and Mama, but often call them Grandma Nana and Grandma Mama.
Grandpa Bill is (xh) Dad and Grandpa David is Mama's second husband (not Grandma Roberts second husband).