I understand your situation from a flip side. I have two boys and one of them is not my husbands but from a previous marriage. When my ex husband married his new wife he moved away and hasn't had much to do with my little boy, but at first I thought they might have a relationship and my little boy being very young was confused about her kids calling her mama but that I was his mama. We agreed he could call her
Mama Arleigh, which is her name. It didn't bother me cuz he knew who his mama was and it made her feel better not being just Arleigh to him. Try Grandma S., and if they are real small I know they'll make their own name for you anyways just trying to say you name, like "sanny" or something. You're sweet to care about being a part of their lives. It takes a good woman to love not only her own grandkids, but others as well! So I'm sure it will work out! Take care and good luck!
K.