The red ballon

Tonight, the buddies are staying at William's house, and something eerie unfolds as the late night approaches. Just as the children have settled down for the night, they hear a girl crying from the roof. At that moment, William mentions the mysterious attic in his house, which his parents forbid. Unable to contain their curiosity, the children venture up to the attic without William's parents' consent. What could be the source of the sound? What experiences await them there?