You're putting your 6-year-old daughter to bed. You've turned off the lights and now you're leaving the room.
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."