You're putting your 6-year-old daughter to bed. You've turned off the lights and now you're leaving the room.
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This expression means "Good night".
You say "Night night" when speaking to children. Children say "Night night" when speaking with adults too.
You can also use it in a playful way when speaking to someone who you're close with, like if you stay at a friend's house overnight.
When you need to sound a little more adult, say "Good night."