When explaining to someone learning French when one has to use vous (the “formal you” pronoun) or tu (the “informal you” pronoun), there is a basic rule of thumb I find useful:
Vous — Used when speaking to people you usually call by their last name, title, or occupation.
for example: Ms Smith, Sir, Doctor, Officer...
Tu — Used when speaking to people you usually call by their first name, nickname, or [? some word or phrase meaning informal/familiar family nickname ?].
for example: Georges, G-dog, Grandpa...
I’m trying to find a word or short phrase to describe “family nicknames” like Dad, Nana, etc., preferably one that doesn’t include the more formal versions like Father, Grandmother, etc.