John J.
Senior Member.
Interesting. I also grew up saying "night night" as did my parents.
I did too. Probably most of the other English speakers on metabunk did as well. However, I really don't remember reading it unless the character was a child or talking to a child. I may have just not noticed it. The book was Agatha Christie's "Seven Dials", which is populated with upper class British young adults saying clever things. It seemed so out of character for them that it caught my attention.
Same here (in the UK). It had never occurred to me that "night-night" or "nighty-night" might not be a commonplace phrase (in English-speaking countries) until now!
Just an informal / affectionate "goodnight" at bedtime. I don't think there are any class connotations.
There was a BBC TV black comedy called Nighty Night some years ago, https://en.wikipedia.org/wiki/Nighty_Night.