I despise the name granny and if my grand kids called me that I'd slap them...lol. I don't want to be called N./nana either.
I think you should get to be called what you want to be called. If one of our other grandparents wanted to be called granny I'd tell the kids to be polite and call them that but I'd hate it and always call them by their given name when referring to them.
The kids should respect you and call you what you want to be called. It's like the ONLY name we get to pick for yourselves, right?
If they refuse then don't answer when they speak to you. Keep your head turned away and ignore them. If they come up and grab you and start speaking to you and call you that tell them "Honey, my grandma name is XXXXXX, please be nice and use the right name".
This, in my family, would absolutely fall on the parents to teach the kids to refer to me by grandma. If they told the kids they can call me anything and that granny wasn't important then I'd have issues with that parent and tell them off. Then I'd forget what their name was and call them an old fashioned ugly name the rest of the time or until they figured out what my name was.