"The book is read by children" was marked wrong. Should it not be accepted?
"Το βιβλίο διαβάζεται από παιδιά."
Doesn't "The book is being read by the kids" work as a translation?
That is the real problem and not easily understood by non native speakers. There is no difference between simple present ans continuous present in Greek. So we have to use both here, and it is the context that can make the difference.