The case of «das Mädchen» is a mere historical curiosity and doesn't require the memorisation as long as long the person in question is acquainted with basics of the German noun formation, i.e. the "-chen" suffix in German, when added to a noun of masculine or feminine gender, unequivocally results in the "neutralisation" of the noun gender, as well as the "umlautisation" of the stressed vowel of the noun being gender bent, eg. der Hund + "-chen" -> das Hündchen. It's a very simple rule, really.