According to rahyafteha(the missionaries and converts website):”We instruct our children to pray from the age of five, and you should do so from the age of seven. Regarding fasting, we encourage them from the age of seven, to the extent they are able during the day, even if it’s half a day or more or less.
When thirst and hunger prevail over them, they break their fast to get accustomed to fasting and to gain capability. As for you, instruct the young ones to fast from the age of nine, for whatever portion of the day they can manage to fast.
When thirst dominates them, they break their fast.”
 said:)
