Chiisana Koi no Monogatari: Chitchi to Sally Hatsukoi no Shiki

小さな恋のものがたり チッチとサリー初恋の四季

Tiny schoolgirl Chiiko develops a crush on older boy Sally and believes herself to be his girlfriend, although Sally is already involved in a love triangle with a girl called Tonko.

(Source: The Anime Encyclopedia)