Children must sit rear-facing up to at least 12 months and (not or) 20 pounds
American Academy of Pediatrics + National Highway Traffic Safety Administration state that toddlers & preschoolers should remain RF until the maximum limits of the convertible carseat.
Autopsy reports reveal that children under 2 years of age are at 4 times the risk of Internal Decapitation when forward-facing *
Infants & toddlers have a disproportionately large head-to-body size ratio. Young children also have poorly developed, fragile, flexible neck muscles, loose ligaments to allow for growth, small rib cages, undeveloped abdominal muscles, soft spinal columns and unprotected relatively larger abdominal organs. When a forward-facing child’s heavy head is thrust forward in a crash, the child suffers an enormous amount of stress on neck. Imagine that a child's head weighs 5 pounds: in a 30 MPH crash, the child suffers about 150 pounds of force upon the neck. If the spinal cord stretches too far in a crash (a mere 1/4 inch) the child suffers paralysis or death. The young child’s cervical vertebrae are not strong enough to protect the spinal cord adequately when forward-facing in a frontal crash. This is because the vertebrae are in pieces joined only by cartilage. The vertebral arch has not fused into a complete circle of bone, which will enclose and protect the spinal cord. Spinal ossificiation does not begin until about 3 YEARS of age.
Over 90% of crashes are the front & sides of the vehicle. Frontal impacts are by far the most common type of crash + they are more severe than rear impact crashes, but side impacts are more often life-threatening, especially for rear seat occupants & roll over crashes are the most severe type. Rear-facing carseats are safest for all crash types because rear-facing spreads crash forces along the entire back instead of only where the harness touches the body.

Watch this SafeKids news broadcast!
Kids are much more flexible than we think and bent legs are not a safety concern: kids are required to ride rear-facing in Scandinavia till about 50 pounds or 3-5 years old. There is no recorded case of leg injury. Even so, it's comparatively easy to fix a broken leg or foot ... we cannot fix broken necks! Other advantages are that kids are less likely to throw snacks or soft car toys towards the driver + kids cannot kick the driver's seatback, thus reducing driver distraction, the leading cause of crashes.
Print the following AAP Policy Statement to share with your Pediatrician.... Selecting and Using the Most Appropriate Car Safety Seats for Growing Children: Guidelines for Counseling Parents (PEDIATRICS Vol. 109 No. 3 March 2002, pp. 550-553)
* Autopsies studied only included children age 2 years and younger, but the longer we can keep kids rear-facing within the manufacturer's recommended limits, the safer the child is. Especially if the child has health concerns such as Prematurity, Downs Syndrome & any condition in which the child has a larger than average head size and/or slow physical growth. Even adults are safer riding rear-facing when allowed as per the vehicle instruction manual.