"Why aren't children at school?"
Translation:Почему дети не в школе?
I am afraid you are barking the wrong tree: I am not a moderator nor do I have any input into what translations are deemed acceptable. I simply explained the difference between the two words. If you feel your translation should be accepted - report it using the appropriate button.
That said, I personally would not accept it - you are still describing a possible reason, not a purpose of the children's absence. Just replace "because" by "for the purpose of" in your own sentence -- it would sound nonsensical, wouldn't it? That's exactly how using "зачем" sounds in Russian.