Carnal is an adjective meaning "of the flesh." This makes carnal relations a subject that kids want to know more about, but one that both kids and parents may be embarrassed to talk about with each other.
Another use for carnal is to describe something that is worldly (as opposed to spiritual) — "He didn't have much use for religion, preferring the more carnal pursuits of gambling, drinking, and partying." The phrase "carnal knowledge" is often used euphemistically to refer to sexual relations, but the phrase has also been used in the legal sense to describe a specific sex crime.
