flower boy vs girl

flower boy and girl both are nouns.

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

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

Other hypernyms of flower boy include female child, little girl.

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