female child vs girl

female child and girl both are nouns.

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

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

Other synonyms of female child include little girl.

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