According to the instructor pilot, he was beginning a local, instructional flight with a student in his weight-shift-control airplane. During the initial climb, the weight-shift capability of the wing became inadvertently restricted by the student, who was seated in the front seat. Due to the tandem seating arrangement, the instructor was unable to get the student's attention to release control. The student was "younger and stronger" than the instructor. The airplane rolled and impacted hard on the grassy area adjacent to the runway. A Federal Aviation Administration inspector reported that substantial damage resulted to the airframe's tubular structure and the wing. The instructor pilot reported that there were no mechanical problems with the airplane at the time of the accident.