Negligence is, at this point, a valid claim. Whether the reason is bad wood, bad design, improper maintenence, crop circles and alien abduction of staff, whatever, KI certified the ride as safe to run that morning. The ride was not safe to operate. Therefore, KI was negligent in inspecting the ride to ensure safety of guests. If the area that broke was not an inspected area, then the daily inspection procedures are negligent by not completely ensuring that the ride is safe.
To those who believe that these people have no right to sue, I point out, as others have done, the implied warranty of safety.