Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.114B.

Though the young are not required to fast, it is proper for their guardians to encourage them to fast so they will become accustomed to it at an early age. They may fast as long as they are able to and then may break it. ArRabi’a bint Mu’awiyyah reported: “The Messenger of Allah sent a man, on the morning of the day of ‘Ashurah, to the residences of the Ansar, saying: ‘Whoever has spent the morning fasting is to complete his fast. Whoever has not spent this morning fasting should fast for the remainder of the day.’ We fasted after that announcement, as did our young children. We would go to the mosque and make toys stuffed with cotton for them to play with. If one of them started crying due to hunger, we would give them a toy to play with until it was time to eat.” This is related by al-Bukhari and Muslim.