Bifocal and progressive (PAL) lenses are different than the average "single vision" lenses because they allow clear vision at distance and near.
The near prescription present allows the eyes to decrease their accommodative (focusing) response and relax.
There is evidence that the accommodative response is related to the progression of myopia.
Because of this, it makes sense to give myopes a near prescription to help decrease this response. Children typically adapt easily and quickly to these new type of lenses and is a good option to help decrease myopic progression in those that choose not to wear contact lenses.