Response: The fasting of a young (child), as has preceded us, is not obligatory upon him. However, it is upon one who is in charge of his affairs to command him to (fast) and thereby prepare him (for it). And the fast of such a young (child) is (regarded) as a sunnah, in that there is a reward for him fasting, however, there is no sin upon him should he stop.
Shaykh Ibn ‘Uthaymeen
Fataawa Ramadhaan – Volume 1, Page 237, Fatwa No.182;
Fataawa ash-Shaykh Muhammad as-Saalih al-‘Uthaymeen – Volume 1, Pages 493.fatwaonline.com