Lantern

Dropped by Marguerite when you beat her in the Greenhouse (in whichever room you defeat her). "She used its light to control the insects."

Hang it on the lantern scale lock at the end of the Old House 2F hall (the door past the Altar) → the Kid's Room, Detention Room and Hidden Room wing (D-Series Arm).

In Bedroom (Banned Footage Vol. 1) Clancy uses another Lantern: it must hang on its hook before Marguerite comes back, and later goes into the projector box.