"Are you children?"
I think it's because dziećmi is the instrumental case. Please correct me if I'm wrong, but I believe that in Polish, one benefit of the nouns having seven cases is that there are ways to not worry about word order. In English, we have no choice but to say "Are you children?" because we only have one way to say the word "children" and one way to ask "are you?". In Polish, though, the instrumental case of the noun can show that they are the object of this question no matter where you put it. I think you could ask "Dziećmi Jesteście?" and it would be identical Please correct me if I'm wrong!
People tend to claim that Polish has a free word order. That's just not true. We can say that it's 'relatively free' or 'flexible', but definitely not free. And it's not like you can just not worry about it.
Changing word order may result in a sentence that is natural but gives a different emphasis, a sentence that sounds like from poetry, a sentence that technically isn't wrong but is just very weird... or a sentence that is just plain wrong.
"Dziećmi jesteście?" undergoes the "technically not wrong but just very weird" category.