I am complimenting Duolingo with Michel Thomas' Total Italian, and although my answers are not always correct, it has helped me with word order in Italian. At the same time, Duolingo has expanded my vocabulary. Just as in English, not everything follows a rule. Some things you just have to accept as is.
I recently learned in French that "the boys may have an apple" is to be understood as an apple each, even though that is not expressed verbally. Sadly, DL would not let me use "each" in the English translation, but various native speakers assured me in the discussion that the boys do not have to divide one apple between them. Would Italian have a similar usage?
No reason. The general rule is subject + verb. Very often you can drop the subject, except when you need to emphasize it. In this case "vogliono una camicia?" is correct. "Loro vogliono una camicia" can mean something like "they want a shirt? THEY? are you sure?" But there should be emphasis in the stress and intonation too