Luckily, my Grandmothers were called Babcia (Polish) and YiaYia (Greek) so I didn't have a problem (though my Greek Grandmother was really a step-grandmother and I felt weird calling her that).

Anyhow, I would explain to her why you don't want to call her Mom-Mom. Perhaps you and she can come up with another special name? I'm sure she would understand.

Good luck and let us know how it goes.

P.S. Here's a website that may help. I like MiMi. It's sweet.

http://www.janbrett.com/piggybacks/grandma.htm