It's "houpy" (not hopi), from "houpat" (to rock/swing), and it rhymes with "kroupy" (groats). It was still used some 30 years ago, probably still is.
The tongue twister isn't really a tongue twister, it just contains a "ř" (and always next to another consonant) in each word (few Czechs find that difficult to say). And it's 3333 (not 300) silver quails (not swallows) flew over 3333 silver roofs (not a church) :D ... "Tři tisíce tři sta třicet tři stříbrných křepelek přeletělo přes tři tisíce tři sta třicet tři stříbrných střech."
Aparently, the nursery rhyme is longer:
"Houpy houpy, kočka snědla kroupy. Kocour hrách na kamnách, koťata se hněvala, že jim taky nedala." --- (rocking interjections), the cat ate some groats. The tomcat [ate] some peas on the stove; the kittens were upset that she [the cat] didn't give them any [food].