Who decides a child's nickname (not referring to pet names, only nicknames)? The parents/siblings or the child himself.
Child him/herself.
When usually is the nickname decided upon? Birth or before child can talk, or when child decides it himself?
The child decides him/herself.
Where do you live?
East Coast, USA.
I detect, like me, you're endowed with the gift of gab. - "O Brother, Where Art Thou?"