Tsuma, Shougakusei ni Naru.

“A grieving family receives a visit from an elementary schooler claiming to be the mother they lost a decade ago.”
Keisuke Nijima fell into the depths of despair after losing his beloved wife ten years ago to a traffic accident until, one day, an elementary school girl pays him a visit and claims to be his reincarnated late wife. And so the next chapter in a life that was frozen in time begins to turn once again, featuring the most devoted husband and his now elementary school wife!
(Source: Comikey)