shouldn’t the solution be for the regular seat belts be made to adjust?
There are some adults that fit this description here.
IIRC, there is a simple device 1 can buy to clip and slip on the shoulder belt. It goes over the actual shoulder portion, and the vertical length that goes up to the “pulley” (highest point) leading to the shoulder portion. This way, instead of the very-high “pulley” being the driver of the shoulder-portion angle, the device is. You can bring down the angle so it better fits across smaller people.
This would be a better “requirment” (if I believed in universal safety regulations) than the very expensive and elaborate “seats”.