Surely they should accept "I am not a child any more"! I believe the British spell 'anymore' as a separate word...
Indeed. I would never even think to write it as one word like that!
bambina = baby girl?
piu ' is more. non sono is I am not. Here they use that and make piu' anymore. So, "I am a child no more" is awkward and anymore is kind of a twist but better understood.
What is wrong with "I am no more a girl"?
That's not correct grammar in English. It could almost be used like "I am no more (of) a girl than her," but that's comparing oneself to another rather than saying "I am not a girl anymore" like they're trying to get us to say.
or what's wrong with "I am no baby anymore."?