Ruling 141

If a child says an object is impure or he has washed something with water, his word is not to be accepted. However, if a child who is mumayyiz and understands well what purity and impurity are says he has washed something with water, then in the event that the object is at his disposal or one attains confidence in what he says, his word is to be accepted. The same applies if he says an object is impure.