Yep! It's less about the words being spoken, and more about the facial expression and body language and tone of voice.
Works well with other languages as well.
LOL... Although, there's a specific phrase I forget, which translates into the things that become a private language in families. Example:
Peace & Quiet = Cheese & Crackers
Dad would come home and say he wanted some peace and quiet, make himself some cheese and crackers, and sit and read. Kids start calling cheese and crackers 'peace & quiet', and mom and dad start making THEM cheese & crackers when they "ask for them"!
All families have these little code words... that mean something different from what the person is actually saying... but they usually forget about them, even as they're using them, until a nonspeaker of this language is present (babysitter, grandmother, schoolfriend, etc.).