1. On Erev Pesach (the day of the fourteenth) first-born sons fast as a memory of the miracle that they were saved during the Plague of the First-born. This fast is observed by both first-born of a father and first-born of a mother. A young child who is a first-born does not fast;] his father fasts on his behalf. If the father is himself a first-born, and he is therefore required to fast for himself, the mother fasts on behalf of the child (Shulchan Aruch and Mishnah Berurah 470). Some say that the mother is not required to fast, and that the father’s fasting is sufficient for both of them (see Kaf HaChaim ad loc.).
2. If the first-born has a headache or eye-ache, he does not have to fast. Similarly, if fasting is difficult for him, and after the fast he will only be able to eat light foods and only a small quantity of them — it is likely that he will not be able to properly fulfill the mitzvahs of eating matzah and bitter herbs and of drinking the four cups of wine, and so it is better that he not fast. Nevertheless, it is preferable in this case that he eat only dessert type foods as opposed to eating a full meal] (Mishnah Berurah ibid.).
