Let's expand school choice in Oklahoma

"Vouchers are controversial for K–12 education, but they have been widely accepted in the child-care ­world," Douglas J. Besharov and Douglas M. Call write in an article ('The New Kindergarten') in the Autumn 2008 Wilson Quarterly. The authors pay special attention to "the Oklahoma solution":

Using mostly federal funds, the state simply pays child-care centers for a full day for each child, even if the child is only present for four hours. (This practice is documented in government reports, but the folks in Washington either don't know or don't care about it.)

So Oklahoma has school choice for four-year-olds; why not school choice for five-year-olds? We have school choice for 18-year-olds; why not school choice for 17-year-olds?