At what age should children begin to be trained to fast, and how?

Fasting is only obligatory after puberty, but children are encouraged to be trained earlier, step by step according to their ability.
Ar-Rubayyi' bint Mu'awwidh r.a. related that the Companions trained their young children to fast on the day of ʿAshura, and made them toys of wool to distract them when they cried from hunger until it was time to break the fast (Sahih al-Bukhari 1960). Many parents start at around seven with half-day fasts, then full days at weekends, before a full month. Make sure the pre-dawn meal is nourishing and that the child drinks enough when breaking the fast. Do not force a child who is ill or too weak, and do not shame a child who breaks the fast. Praise and a joyful Ramadan atmosphere work better than threats. See Educating the family.