flower girl vs girl

flower girl and girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
flower girl	Yes	No	No	No
girl	Yes	No	No	No

As nouns, girl is a hypernym of flower girl; that is, girl is a word with a broader meaning than flower girl:

Other hypernyms of flower girl include assistant, help, helper, supporter, female child, little girl.

flower girl (noun)	girl (noun)
a young girl who carries flowers in a (wedding) procession	a young female
a woman who sells flowers in the street	a girl or young woman with whom a man is romantically involved
	a youthful female person
	a friendly informal reference to a grown woman
	a female human offspring