Laws of Public Fast Days
9. There is another lenient dimension to the first three fasts [as opposed
to Tisha B'Av]: Pregnant and nursing women who experience discomfort are
not required to fast (1). Similarly, a person who is sick, even though he
is not dangerously ill ("choleh she'ein bo sakana"), should not fast (2).
Nevertheless, those who are allowed to eat should not indulge themselves in
delicacies and should eat only what is necessary to maintain their body's
health. Similarly, children, although they are not obligated to fast,
nevertheless, if they have the maturity to appreciate [the need for]
mourning, they should be trained (3) to mourn together with the community
and should be fed only bread and water (4).
(1) Even if they are not experiencing discomfort, they are exempt from
fasting, because when the Jewish people accepted these fast days upon
themselves, pregnant and nursing women were not included in the obligation;
however, throughout the generations, pregnant and nursing women have been
stringent and have accepted these fasts upon themselves. If they are
feeling weak ("chulsha"), they should not fast; if they are experiencing
great discomfort ("mista'aros harbei"), they are prohibited from fasting
(Mishna Berura 550:2 and 5; see also Sha'ar HaTziyun 550:3).
(2) It is forbidden for him to fast (Mishna Berura 550:4).
(3) But there is no OBLIGATION for "Chinuch" (training) in this area (ibid).
(4) Or other simple foods (ibid).