Here's one possible answer, courtesy of Applied Ergonomics.
Researchers at The University of Sheffield in England compared the amount of vibration in the drivers' seats of cars to the amount of vibration in child safety seats. Vibration is the most common cause of discomfort during travel, and it's the reason truck drivers and train engineers are likely to have back problems.
They found that while the driver's seat was designed to minimize vibration, the child seats actually multiplied the amount of vibration to which the kids were subjected. Some seats were better than others, but none was really comfortable.