flower girl vs female child

flower girl and female child both are nouns.

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

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

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

flower girl (noun)	female child (noun)
a young girl who carries flowers in a (wedding) procession	a youthful female person
a woman who sells flowers in the street