Rumi-chan's Classroom

(from ebookjapan):

Rumi lives with her brother in a shabby apartment. Although she is not rich, she lives a very happy and joyful life.

Even though she has not yet started school, she follows her older sister to the fourth grade classroom every day. This is a heartwarming story about the adorable Rumi and the people around her.

This is a legendary rare work that is said to be the pinnacle of children's manga.