girl vs female child

girl and female child both are nouns.

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

As nouns, girl and female child are synonyms defined as:

Other synonyms of girl include little girl.

girl (noun)	female child (noun)
a young female	a youthful female person
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