I'm Grandma, (or Maama, as my 2yr old DGS calls me), my son-in-law's stepmother (who raised him since he was 5) is Nana, and his biological mother is YaYa (she's Greek and that's what they say).
My brother's grandkids call him Bop and my sister-in-law is MiMi.
Are you Irish? German? French? etc. Maybe you could use their version of Grandma.
Congratulations BTW and the kids will love you no matter what they call you if you treat them as a Grandma does.