996. When Children Start to Fast

Shvisas Asor 2:9

If a pregnant woman smells food and craves it, we should whisper in her ear that it is Yom Kippur. If this proves sufficient to settle her mind, that is satisfactory. If this is not sufficient to calm her, then she should be fed until her craving stops. Similarly, if a person is overcome by an overwhelming hunger that endangers him, he should be fed until his vision clears up. This should be done immediately, even if it requires giving him meat that was not ritually slaughtered or that came from non-kosher animals. We do not make him wait for kosher food to become available.

Shvisas Asor 2:10

Once a child turns nine or ten years old, he should be trained to fast for a few hours. For example, if he usually eats two hours after daybreak, on Yom Kippur he should be fed in the third hour. If he usually eats after three hours, he should be fed in the fourth. We impose fasting on a child according to his stamina, increasing it accordingly. Once a child turns 11, it is a rabbinic enactment that he complete the fast in order to be trained in the mitzvah. This is true of both boys and girls.