Should "Montre-moi les mains!" really be considered wrong? I understand you put that in this lesson as an example of reducing ambiguity, with "tes mains", but I definitely don't see it as something to be taken as a wrong answer in a quiz.
If I'm correct, we do the exact same thing in Spanish, and both "Muéstrame tus manos" y "Muéstrame las manos" would be correct. There is no ambiguity whatsoever (i.e. no sane person would wonder whose hands we're asking the person to show). Is it really really different in French?
I mean, it is one thing to try to get students to answer what you taught them, and a very different thing to reject right answers (especially when this very same lesson covers using definite articles for this).