There is no Kaffarah (expiation) on you. Similarly, if your son has not experienced any of the characteristics of puberty such as discharging of semen, growing of rough pubic hair, he is neither accountable nor obliged to pay Kaffarah. In this case
the Diyah is to be paid by his family. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.