"Are you children?"
Definitely not as a translation of "Are you children?", but if we made both the English and Polish sentences longer, there are some options where both can be beginnings of sentences and still match each other grammatically and semantically: "Are you, children, hungry?" - "Czy wy, dzieci, jesteście głodne?" I'm not 100% sure if the commas are needed, but I think they're a safer option to make those sentences perfectly grammatical.
Most of the English questions being translations to some "Czy wy, dzieci" questions would need to start with "Do you", and not "Are you", though.
I don't think that the phrase "czy wy dzieci" is too popular, sounds a bit old-fashioned, and google gives less than a 1000 results for the exact phrase. Although that may be because an adjective could often be used: "Czy wy, drogie dzieci..." (Are you, dear children...")
You need to conjugate your verbs, polish is a case based language and,sometime direct translations dont work jesteście is you are(plural). Czy is used in a way where you may already know the answer to the question and you just asking to be sure. Using czy and not using czy in your quest is the difference between "a lion is a cat?" And "isn't a lion a cat?"
„Jesteś” = second person singular
„Dziećmi” = Instrumental plural
Either „Czy jesteś dzieckiem”('Are you a child') or „Czy jesteście dziećmi”('Are you[y'all] children') and since here you are supposed to translate 'Are you children' only the latter is acceptable. ;)
Good example for 2nd person plural! I imagine it being very difficult to learn these things by heart if they are non-existent in one's native tongue.