My kids call my inlaws Grandma and Grandpa.....although DS14 calls them G-ma and G-pa which is what all the teenagers call them. They even sign cards and gifts that way.
With my birth family, I only met my paternal grandmother once so never called her anything. My maternal grandmother is called Gramy. Both of my grandfathers passed away before I was born.
My kids call my maternal grandmother Great-Gramy, my birth father Granddad and my birth mother Nona (which is what she wanted to be called).
With my foster family, I called my paternal grandmother Grandma and my paternal grandfather Pop-pop. My maternal grandfather passed away before I was born but my siblings called him Granddaddy. My maternal grandmother is calld Mommom (pronounced Mum-mum).
My kids call my Mommom "Big Mommom" and my foster mom "Little Mommom". However, they are the only ones who call my foster mom by that name. When my siblings and I had kids, she let the firstborn child decide what that family would call her....so she is called Gammy, Mammy, Mamaw, Grammy and Little Mommom. Kind of confusing when we're singing happy birthday to her but it works! :~)
I don't have grandchildren yet but I like the way Mommom sounds....although one of my cousins calls her grandmother "Honey" which I think is so sweet.