A nick name is something someone acquires in life because of a personality trait, or characteristic. Ie Spud (cos his head is shaped like a potato), Doughnut (because he was asked to go on a snack run for 3 people and came back with 24 doughnuts) etc etc.
If you give your child a name that can be shortened, it's a derivative or shortened name. Not a nick name. IE name them William, shortened to Will, Bill, Billy. Elizabeth shortened to Liz, Beth, Libby.
Drives me insane!
See me same time next week re Staycations!!