Reference: http://mufti.af.org.sa/ar/content/متى-يأمر-الأطفال-في-الصيام
Question: When are the young children required to fast?
Response: The young are to be encouraged to fast when they are 7 or 8 [years of age], and like this they will develop a habit [to fast] by the time they reach ten [years of age].
The companions [of the Prophet ﷺ] used to encourage their children to fast, and they would give them toys [made] of wool to distract them [from their fast]. So one should develop their habit [to fast] when they are young, before puberty by about two years [or so], and it is anticipated they will develop a desire for it [in shaa.-Allaah].
