female vs female child

female and female child both are nouns.

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

As nouns, female child is a hyponym of female; that is, female child is a word with a more specific, narrower meaning than female:

female (noun)	female child (noun)
an animal that produces gametes (ova) that can be fertilized by male gametes (spermatozoa)	a youthful female person
a person who belongs to the sex that can have babies

female (adjective)	female child (adjective)
being the sex (of plant or animal) that produces fertilizable gametes (ova) from which offspring develop
for or pertaining to or composed of women or girls
characteristic of or peculiar to a woman