Jo sounds good to me. It won't take away from your niece's special (?) name for you, and doesn't sound like it could be a grandmother's nickname to me like
Nonie does. Other options could be
Annie, or something organically chosen by your godchild based on something he/she finds especially profound about you. Or your grandchild could surprise you by insisting on calling you Joanie.
By the way, I love your sig pic! I remember playing that version of
The Oregon Trail when I was but a wee tot in the hospital. It was the only decent computer game available to me at that time, so I have fond memories of it.