Mugen Densetsu Takamagahara

A fifth-grade girl named Yuuki Wakasa is responsible for caring for her three younger brothers. While walking home, she finds a red magic stone that falls from the sky. Looking in the mirror, she sees a dark-haired girl who explains that the stone grants the ability to travel between the human world and a dream world called Takamagahara. Yuuki learns she is the chosen one tasked with saving the Sun in the dream world to prevent both realms from falling into eternal darkness.