The exercise i did was a choose the box exercise. "You are child" was the only "acceptable" choice. It should have had an "a" in the English translation, and even when it came up as the answer, it said the correct answer is "you are child". Just wanted to point out this minor error. Maybe it can be corrected? I don't really care, as I understood it, but it would've been marked wrong in my elementary English lessons as a child.
Well I think it depends on the context. In my opinion, Tu ești copil could mean the following: 1. A statement - you are a child 2. A question - Are you a child? 3. An insult - Are you a child (to act like this)?
Usually the intonation makes the difference between a statement, a question and an insult. If you use like a mocking tone or one that could imply sarcasm or something, than it could be used as an insult.