I'm not sure you always get to choose. Kids have a way of saying something before they talk well and then it sticks. I know a lady called "Mammy" because her first grandchild couldn't say Mamaw. She didn't love it at first, but now the great grandkids call her that, too.
My kids call my mother Mamaw. They call my father Old Bob. He started that when the first one was born and it stuck. He has grandkids from 4 to 25 calling him Old Bob.