girl vs flower girl

girl and flower girl both are nouns.

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

As nouns, flower girl is a hyponym of girl; that is, flower girl is a word with a more specific, narrower meaning than girl:

Other hyponyms of girl include Campfire Girl, farm girl, moppet, schoolgirl, Scout, flower boy.

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