During a private pilot practical examination, the private pilot candidate was seated in the left front seat of the airplane at the controls, with the designated pilot examiner candidate seated in the right front seat. The examiner candidate was being evaluated by a designated pilot examiner seated in the left, second row seat behind the pilot candidate. According to the examiner candidate, the flight segments were met satisfactorily and the pilot candidate was asked to complete a crosswind landing. The examiner candidate reported that the pilot candidate struggled during the first approach and initiated a go-around. During the second approach, about 15 feet above the runway, the pilot candidate pulled the throttle off. According to the pilot examiner seated in the back, the examiner candidate reached for the throttle as the airplane landed firm. The examiner candidate completed the landing and informed the pilot candidate that the examination was unsatisfactory and taxied the airplane to the maintenance hangar. An examination of the airplane revealed that the airplane sustained substantial damage to the firewall. The pilots reported that there were no mechanical failures or anomalies with the airplane prior to or during the flight that would have prevented normal flight operation.