I just thought I would add my experience with my kids and grandparent's names.
My ODS started calling my dad Papa at a very young age. We were planning on calling him Grandpa but the the Papa stuck. I sometimes call my dad Papa and I think that is partly where he got it from and partly because his older cousin calls his OTHER grandpa Papa. However, what is funny is that rather than calling his other grandpa Papa - like his cousin did - he calls him Papapa. Hee. Guess my son feels the need to be an individual.
That same nephew calls his other grandpa (so my SIL's FIL) [Name]Papa - something that he came up with when he was little. Super cute! In fact, everyone - including my boys - refer to him as [Name]Papa.
Interestingly, both Grandmas are just Grandma [First Name].