The kids were blind-folded while playing the game to make it more challenging.

blind-folded 是动词 blindfold 的过去分词形式，在句中表示被动。blindfold 由 blind（盲的）和 fold（折叠）组成，字面意思是"把眼睛折叠起来"，即"蒙上眼睛"。blind-folded 表示"被蒙上眼睛的"。