sex kitten vs girl

sex kitten and girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
sex kitten	Yes	No	No	No
girl	Yes	No	No	No

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

Other hypernyms of sex kitten include fille, miss, missy, young lady, young woman.

sex kitten (noun)	girl (noun)
a young woman who is thought to have sex appeal	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