When can we say nighty night?

Question

I watched a movie.It was naptime in the afternoon for a young child. I heard her brother said nighty night. I think we can say it only at night.

Can we say it at naptime?

And what should we say to children before naptime?

Answer 1

Note that "nighty night" is something that you normally only say to a small child. If you said "nighty night" to your boss as you were leaving work this would be considered very strange, regardless of the time of day.

That said, it makes most sense when the child is going to bed for the night. People do often use it when a child is taking a nap, even if that's in the middle of the day.

Answer 2

According to Oxford Advanced American Dictionary,

nighty/night night
used by children or to children, to mean “Good night”
“Nighty night, sleep tight!”

So you can say "Nighty night" to children the same way you say "Good night" in typical conversations.

According to my experience, you can say "Good night!" from the evening onward as a farewell, when you don't expect to see the other person again that day. (When the evening starts, in my opinion, is rather subjective. Usually, most people seem to think it's around 5pm or 6pm.) In practice, it's rather unclear that you can say this in the afternoon, but I believe you can, as a farewell.

For further information on "Good night", I suggest reading this question.
(As a side note, "Good evening" is for greeting only.)