Nabari no Ou dj - Osanagokoro no Niwa

Set between chapter 56 and chapter 57 of Nabari no Ou.

Miharu asks Yukimi where his childhood self has gone to. He sets off to the abandoned train while thinking about it, and gets off at the Garden of Chidren's Hearts, a garden completely separated from the world. Could he find what he is searching for there?