The correct option is C It was the innocent smile of a child at having found something new and unique.
Options A and D features sentences where the predicate comes before the subject. Therefore these are not appropriate. Option B indicates that the discovery of the child's smile turned something into new and unique. However, it is because of the uniqueness of the thing that the child smiled innocently. Therefore option B is also incorrect, and option C being closest to the meaning, is correct.