Words ending with -chen are always neutral. It is an affectionate ending, like Maus + mouse, Mauschen + Mousey. I hope you get it. We don't know why the world 'girl' has this in German. I only assume that very long ago there was a word like "Die Mad", like 'maid' in English. So if you think about that, das Madchen means something like 'the little girl'