Children who have yet to reach the age at which they are obligated in the mitzvot are exempt from the fasts that the Rabbis instituted. And the Sages did not require us to train our children to fast for a few hours; they did so only in regard to Yom Kippur, which is Torah-based… Nonetheless, many have a custom to train their children to fast a few hours, each one according to his or her strength. But children should not fast all day long (Rama of Panow 111; see KHC 554:23). When feeding children on a fast day, one should give them only simple foods, in order to teach them to mourn with the congregation (MB 550:5).
Brides and grooms are obligated to fast on the minor fast days. Even though they have a mitzvah to rejoice for seven days after their wedding, and they are therefore forbidden to accept upon themselves a private fast, nonetheless, they must observe public fasts, because public mourning overrides private joy. Moreover, brides and grooms have a special mitzvah to remember the churban (the destruction of the Temple), as it says (Tehillim 137:6), If I fail to raise Jerusalem above my highest joy (Ritva, BH 549:1. Many authorities are lenient regarding Ta’anit Esther; see below, chap. 14, n. 12).
The main participants of a brit milah (circumcision) – namely, the father, the sandak, and the mohel – are also obligated to fast. The same is true of a father who redeems his firstborn son (pidyon ha-ben) on a fast day; he may not eat. Instead, the custom is to perform the brit or the pidyon towards the end of the fast and have the [festive] meal after the stars emerge.
Soldiers who are engaged in a defensive operation that is liable to be compromised if they fast should eat and drink as usual so that they can carry out their mission properly. However, soldiers who are [merely] training must fast.