Try not to place so much emphasis on it. I have no children of my own but have step children, one set of grandchildren call me Grandma, the other call me Mamma (a local term). Neither would have been my choice but I didn’t feel I could say anything as I was grateful that my stepsons counted me as the grandmother to their children. However, when you have a small person joyously flinging themselves at you shouting “Mamma” or writing little notes saying that they ‘love you to the moon and back’, it matters not a jot.
Also, they may have entirely different ideas. My late Mother wanted to be ‘Granny’.when the eldest was a toddler she called my Mum “Gan Gan” and it stuck. All six referred to her as ‘Gan Gan’ for the rest of her life and still talk about ‘Gan Gan’ although they are all in their 20s.
Congratulations by the way, don’t worry too much and I hope you will be being a grandmother as much as I do ?