girl vs flower boy

girl and flower boy both are nouns.

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

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

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

girl (noun)	flower boy (noun)
a young female	a young boy who carries flowers in a (wedding) procession
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