Question: When should we start training our children to fast?
Answer: The Gemara (Yoma 82a) writes that we start training children to fast for ‘hours’ before they are bar mitzva or bas mitzva. Thus, the Shulchan Aruch (OC 616:2) writes that when a child reaches the age of nine, we begin to educate them about fasting by feeding them a little later than usual on Yom Kippur. When they reach 11, there is a rabbinic obligation to fast the whole day. The Rema, however, disagrees, writing that there is no such obligation.
Nonetheless, the Bach (OC 616:6) demonstrates from the Gemara that children who study all day are considered to be weak and therefore have the status of a choleh who doesn’t fast. This justifies the practice of children not fasting before they are bar mitzva or bas mitzva (See Kaf Hachaim OC 616:16). Likewise, the Mishna Berura (616:9) writes that children nowadays should not fast before they are bar mitzva or bas mitzva as they are assumed to be weak. Only if one can ascertain that the child is fit to fast, may they do so.
The Mateh Ephraim (Elef Hamagen 616:5) criticizes those who don’t feed their younger children as usual on Yom Kippur night.
While many children observe the three fasts before their bar mitzva and bas mitzva, R’ Shlomo Zalman (Halichos Shlomo, Yom Kippur 6:n67) maintained that this custom is erroneous and has no source.
The Mishna Berura (550:5) writes that there is no obligation for children to fast for a few hours on the other fasts, either, though they shouldn’t eat too much.
R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 39:31) writes that a choleh or child who eats on Yom Kippur does not make kiddush before eating. Nor is there a requirement for lechem mishne. Quoting R’ Shlomo Zalman Auerbach, he writes that they should, however, wash their hands until the wrist before eating bread.In conclusion, 11 year old girls and 12 year old boys should ideally eat breakfast a little later on Yom Kippur. There is no obligation to train children to fast at all before their bar mitzva or bas mitzva for tisha b’av or the other minor fasts.