I cannot see well how καλὰ καὶ ἀγαθά is being used here, is it part of the direct object or a modifier of it?
I am almost sure it is modifying it, because if it was the direct object then τοὺς πολίτας would only make sense as an indirect object. I would render the sentence, then, as: 'The poets used to educate the good and beautiful citizens'. But I am not sure, what do you think?