This is a popular classical piece named Duett buffo di due gatti (humorous duet for two cats). all the lyrics made of the word miau (meow) and this classic comic piece is still widely performed.
The group performing this classical piece named Les Petit Chanteurs A La Croix De Bois (Little Singers Of Paris). The boys in the video are part of this group Their names are Hyacinthe de Moulins and Regis Mengus and now they are all grown up.