Reference: Fataawa Ramadhaan – Volume 1, Page 237, Fatwa No.182
Fataawa ash-Shaykh Muhammad as-Saalih al-‘Uthaymeen – Volume 1, Pages 493
Question: What is the ruling of the fast of a young [child] who has not yet reached puberty?
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.