Dang, I'm losing track of all these relatives! (-: I thought you were being a niece. No, that's right, you're a granddaughter; now I remember thinking, "Lady of Narnia rhymes with Danny Darnia, another of my granddaughters." Although the term Papa was never used for grandfathers when I was a kid, it is now, and Amanda uses that with me. So "Papa Joe" is perfectly suitable. Or you could say "Grandad," as Dani does with me.