The living. Those who still enjoy the light of day. The repetition is emphatic, and has the force of "the living, and the living only." The father to the children. Hezekiah may, or may not, have had children himself at the time.
Manasseh was not born; but he may have had daughters, or even other sons, who did not survive him. He is not, however, perhaps, thinking of his own ease.