Your father/mother's father is your grandfather or grandpa, and you are his grandchild or grandson/granddaughter.
Your father/mother's mother is your grandmother or grandma, and you are her grandchild or grandson/granddaughter. (Sometimes we say grandkids for short, though calling one child a grandkid is fairly uncommon to my knowledge).
Your father/mother's brother is your uncle, and you are his nephew/niece.
Your father/mother's sister is your aunt, and you are her nephew/niece.
So Orda is talking about grandchildren (his son's children). So he could also say "I'll be a grandpa" if that will fit better.
I'd avoid him being a "granny", cause I associate grannies with little old
ladies specifically, and I know that's the common definition where I'm from at least.