To be completely honest, I can't even remember how my kids started calling their grandparents the names they're called.
My kids are the first grandchildren on both sides of the family, so there wasn't anything to really go off of. I think my son had a lot to do with the choosing of names.
-My mom and stepdad: Nana Sandie and Papa
-My dad and stepmom: Grandpa Roger and Ronni (maybe Grandma Ronni, to be honest, I'm not entirely sure...I don't spend hardly any time around my stepmom)
-MIL and FIL: Grandma Kris and Grandpa Bob
-My grandparents: Great Nana and Papa
-FOBs grandparents: Great grandma and Great grandpa.
Kind of off topic, but my son pretty much got to decide what we called his aunts and uncles. For example, he calls my SIL TeeTee, because when he learned to talk, Auntie Tracy was too hard for him to pronounce and it just kind of blended into TeeTee.