carnal abuse vs sexual activity

carnal abuse and sexual activity both are nouns.

Word	Noun	Adjective	Verb	Adverb
carnal abuse	Yes	No	No	No
sexual activity	Yes	No	No	No

As nouns, sexual activity is a hypernym of carnal abuse; that is, sexual activity is a word with a broader meaning than carnal abuse:

carnal abuse: any lascivious contact by an adult with the sexual organs of a child (especially not involving sexual intercourse)
sexual activity: activities associated with sexual intercourse

Other hypernyms of carnal abuse include sex, sex activity, sexual practice.

carnal abuse (noun)	sexual activity (noun)
sexual intercourse with a person (girl or boy) who has not reached the age of consent (even if both parties participate willingly)	activities associated with sexual intercourse
any lascivious contact by an adult with the sexual organs of a child (especially not involving sexual intercourse)