I live with a two year old and a four year old. This is my story, and I am sticking to it.
My grandchildren call me Bapa. It is a name I am told that I created when I was trying to call my paternal grandfather something he would recognize. I apparently blurted out Bapa, and it stuck. I consider it an honor to carry on that name as my Bapa was a great man.
My wife is Nana. For my son, the name Nana is a family name. Lots of families use it, but it has special significance to my son. So that is what the kids call my wife.
They call me Bapa, and they call my wife Nana ... over and over and over and over and over and over and over again.