Here is what I found:
FAA believes that forward facing child restraint devices are superior to vest- and harness-type devices, booster seats, belly belts, and the holding of children on laps. The FAA recommends:
the use of a rear facing child restraint system for children weighing under 20 pounds;
and a forward facing system for children weighing between 20 and 40 pounds;
Children weighing over 40 pounds should use the standard lap belt that is attached to all airline seats.
hope that helps