Just as with a car, a lap belt is not sufficient protection for a small body.
This is not true. A car is not like an airplane. Most car crashes involve stopping suddenly, from moving horizontally to not moving at all.
In turbulence, an airplane can move up and down or left and right quickly, but not horizontally. A shoulder belt does not help much in this situation.
In a survivable crash, the plane doesn't stop suddenly. It slides or rolls along the ground until friction slows your speed down to zero.
What I'm getting at is that many people (including Congress) equate driving in a car with flying on a plane in terms of safety and crash worthiness. If the same program of crash prevention (as opposed to crash-worthiness) that is used for planes were applied to cars, driving would be much safer.