I choose to believe there’s a missing panel, where the mirror tells the queen that it is simply finding the most attractive person by the person asking its own standards. If you ask a magic mirror who the most attractive person is, it will show you the most attractive person per your beliefs. If they’re a minor, that’s on you. Isn’t that exactly what a magic mirror would do, reflect your beliefs and inner desires back upon you?

And for pointing that out, that’s why the mirror was sent to prison.