knee pants vs britches

knee pants and britches both are nouns.

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

As nouns, britches is a hyponym of knee pants; that is, britches is a word with a more specific, narrower meaning than knee pants:

Other hyponyms of knee pants include buckskins, plus fours, trunk hose.

knee pants (noun)	britches (noun)
(used in the plural) trousers ending above the knee	(used in the plural) informal term for breeches