Q: At what age should children begin fasting on Yom Kippur, for either part of the day or the entire day?

Rabbi Eli Mansour:  Chacham Ovadia Yosef addresses this issue at great length in Chazon Ovadia (Yamim Nora'im, p. 330), and he rules that children younger than nine years of age should not be forced or even allowed to fast for any part of the day.  Even on the night of Yom Kippur, when they return from the synagogue, children are allowed to eat and drink as usual.  Chacham Ovadia very adamantly insists that parents feed these children and have them drink as they normally would, as even several hours of fasting could potentially endanger their health.

Once a child reaches the age of nine, parents should begin training them to observe "Ta'anit Sha'ot," meaning, to fast for a brief period of the day.  If, for example, a child normally eats breakfast at 8 A.M., the parents should feed him one hour or so later (and he should not eat or drink on the night of Yom Kippur).  Parents must ensure that the child eats properly during the day.  If children attend the synagogue services on Yom Kippur, which generally end only at around 1 or 2 o'clock in the afternoon, parents are required to either send the children home to eat or have food for them in the synagogue so that they are properly fed.  In fact, Rabbi Chayim Palachi (Turkey, 19th century) would make an announcement in his synagogue before Torah reading on Yom Kippur reminding parents to feed their children, lest the children go unfed until the afternoon hours.

Chacham Ovadia Yosef also emphasizes that this applies only to healthy children.  If a child has any sort of medical condition or is feeling ill or frail, he must be given adequate food and drink on his usual schedule, and should not be allowed to fast at all.

Within a year of a child's Bar/Bat-Mitzva, meaning, at the age of eleven for girls and twelve for boys, the child should fast until Chatzot (midday as defined by Halacha; the time appears on most Halachic calendars).  Chacham Ovadia Yosef rules that given the frailty of today's children they should not be allowed to observe a full day of fasting until the age of Bar/Bat-Mitzva.  Therefore, eleven-year-old girls and twelve-year-old boys should fast only until Chatzot and no later.

Summary: Children under the age of nine must be fed and given drinks according to their normal schedule on Yom Kippur; it is forbidden for parents to allow children of this age to fast for even a brief period.  From the age of nine, healthy children should be fed an hour or so later than their usual mealtime, and parents must ensure not to withhold food or drink from these children any longer than this period.  Sick or weak children at this age should be fed as usual and must not fast for even a brief period.  Within a year of a child's Bar/Bat-Mitzva, he/she should fast until Chatzot; a child should not observe a full fast until reaching Bar/Bat-Mitzva.