knee pants vs trousers

knee pants and trousers both are nouns.

Word	Noun	Adjective	Verb	Adverb
knee pants	Yes	No	No	No
trousers	Yes	No	No	No

As nouns, trousers is a hypernym of knee pants; that is, trousers is a word with a broader meaning than knee pants:

knee pants: (used in the plural) trousers ending above the knee
trousers: (usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately

Other hypernyms of knee pants include pair of trousers, pant.

knee pants (noun)	trousers (noun)
(used in the plural) trousers ending above the knee	(usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately