Its in the library when you get to the kids reading room, The moment you enter the room turn to your left and try to open the door. Shoot your capture stream into the crack in the doo to capture the chair and try and use it to smash whatever is blocking the door. Once you do this pull out your PKE meter and go all the way to the left corner of the room. The child ghost will fly off of the chair, turn around and leave the small room, then look for small footprints and a ghostly hopscoch on the floor in the center aisle. Follow the fooprint to a puppet show infront of black slime covered walls and then you should get the trophy.