my guess of the story (spoiler)

the 4 demons are actually taking the 5th fox demon in custody (for some unknown reason). look at the poster: 4 demons are surrounding the fox demon who clearly has cuffs on

somehow the fox demon can't run away by itself, but it can be released/replaced. the 4 demons need kids' life source and the fox demon is responsible for attracting the kids.

the kid who survives the hide-and-seek eventually releases the fox demon (into real world?) and becomes the next to-be-released fox demon, while those dead ones become the batteries.

and hence begins the next round of the game.

questions about kid-batteries:
those 4 on the pagoda shows number 6 through 9, Yaimo has number 0, the kid gangster boss is number ?? outside (i can't read that number)
where are slot 1-5 and what are they for? (coincidently, there are 5 demons there)