Reprinted from Aish Thornhill Community Shul by Rabbi Rothman

Who is considered a first born in regard to the Fast of the First Born (ta’anis bechorim) on erev Pesach?
Any first born male who is over thirty days old, whether first-born to his father or to his mother, is considered a bechor. A first-born of a kohen or a levi is considered like any other bechor in regard to this fast. A bechor born by cesarean section, or of a first-born non-Jew who converts is a matter of disagreement among the poskim. It is recommended that these bechorim participate in the siyum.
It is customary that a father fast for a young child and even should do so if his wife is pregnant with their first child.
Must the bechorim attending the siyum actually hear the Gemorah being completed? Must the bechorim eat at the siyum?
It has become customary for the bechorim to exempt themselves from fasting by participating in a siyum and eating what is served: The bechorim attend a siyum and listen as the tractate is completed. Food is then served and eaten by the participants.
If a bechor did not hear the tractate being completed because he came late, if he eats at the siyum then it is considered as if he fully participated.
Similarly, if one hears the siyum, but does not partake of the food being served he is, as well, considered as if he participated fully.
Obviously, in order to fulfill the requirements of all the poskim, one should l'chatchilah listen and understand the proceedings, and partake of the siyum meal. By only doing one of the two, it is not prefered, however, the person has fulfilled the requirement.