10-Year-Old Mother

Wauman (Nagomi) who lost her mother due to a water accident at the age of 5. A decade later, a mysterious primary school student Mr. Ano Mari appears before him. She said that she is a reincarnation of her mother and starts to live with Wakami. Two new parent-child relationships are established.
From Comic Flapper.
Note: this manga was cancelled/discontinued due to the death of the author.