Hacham Ovadia Yosef, in his work Hazon Ovadia (Yamim Nora’im, p. 330 and onward), writes that children under the age of nine should not fast at all during Yom Kippur; parents should ensure that children at this age eat according to their normal routine. If a young child wishes to fast even for part of Yom Kippur, the parents should object and see to it that he eats normally.
Once a child reaches the age of nine, however, he should observe a "Ta’anit Sha’ot," which means that he eats an hour or two later than he normally does. Thus, if a child normally eats breakfast at 8 AM, then starting from the age of nine, he should eat breakfast at 9 AM or 10 AM, so that he begins being trained in this Misva. Hacham Ovadia adds that parents of children this age must ensure that they eat and drink properly on Ereb Yom Kippur, so that they will be able to fast for an hour or two on Yom Kippur without any risk to their health. He also writes that if a child of this age wakes up during the night and wishes to drink, his parents may give him a drink.
This procedure should be followed until the child reaches the age of Misva obligation – twelve years for a girl, thirteen years for a boy. Hacham Ovadia maintains that nowadays, when children do not possess the same physical strength as in previous generations, children should not fast the entire day until they reach the age of Misva obligation; in his view, a twelve or eleven-year-old child should not fast past midday. Parents must assume responsibility for the physical well-being of their children and ensure that they eat and drink sufficiently on Yom Kippur, so that they do not risk their health, Heaven forbid.
Of course, once a child reaches the age of Misva obligation, he or she is required to observe the complete fast. If there is some concern regarding a child’s ability to complete the fast even at this age, a physician must be consulted ahead of time to determine whether he or she is capable of fasting without posing a health risk. Hacham Ovadia emphasizes that one must not be more stringent than Halacha requires. The principle of Piku’ah Nefesh (concern for human life) overrides all other Halachic concerns, and one should therefore be more stringent with regard to Piku’ah Nefesh than with regard to the Yom Kippur fast. Hacham Ovadia writes that even if there is a slight possibility of a health risk, the individual in question should not fast. Parents must therefore exercise care and ensure before Yom Kippur that their Bar-Misva and Bat-Misva-age children are physically capable of fasting.
Summary: Children below the age of nine should eat and drink normally on Yom Kippur. Once a child reaches the age of nine, he or she should eat on Yom Kippur morning an hour or two later than usual. A child should not be allowed to fast beyond midday on Yom Kippur until he or she has reached the age of Misva obligation (twelve for a girl, thirteen for a boy). Once a child has reached the age of Misva observance, he or she must complete the fast, but parents must ascertain ahead of time that the child is physically capable of fasting without subjecting himself or herself to any risk. If there is any medical risk involved, then the child should not fast, as the concern for human life overrides the Yom Kippur fast.
