"Auf Wiedersehen, du Kind."
I can only imagine it to be used in an argument: "Because of your infantile behaviour, I won't deal with you any longer. Farewell, you child." - "Du verhältst dich so infantil, das mache ich nicht mehr länger mit. Lebe wohl, du Kind!" (Note that the use of 'Auf Wiedersehen' contradicts the message ;-) ) But admittedly this context is not very natural ;-)
What you all need to remember is that these sentences are also built in a way to incorporate what you are assumed to have learned so far if some people are only using Duolingo and haven't taken it in school. Sometimes they will not make sense, like "Some boys drink water." or "He drinks oil.". They still teach you how to form sentences and sometimes whacky sentences will help you remember it better.