Great question — answering as the UphillTech team:
The movie, if I'm not mistaken, was a Halloween special that aired once and then never again called Kakurenbo aka Kakurenbo: Hide & Seek. A young boy named Hikora enters the ruins of a forbidden city with a group of other children to play "Otokoyo" (a game of hide-and-seek where the players are said to be kidnapped by ghosts and demons) to find his missing sister. - IMDb
Full answer by user20985: https://movies.stackexchange.com/a/33968 (CC BY-SA 3.0).
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