Airplanes are not like automobiles. Shoulder belts are helpful in cars if you stop suddenly (as in, crashing into another car or a wall). If an airplane stops suddenly, it means you're dead.

Seat belts on airplanes are quite useful for combating the effects of turbulence. A simple lap belt on a child or an adult will do the trick (i.e., you stay stuck to the seat instead of flying around the cabin).

In the event of a survivable crash, it will take me longer to take my child out of a car seat than to unbuckle a lap belt. Time is precious in that circumstance.