The Rama in Orach Chaim 470:2 says that a father fasts for his son who is a Bechor until he grows up. The common Minhag is that the Bechorim go to a Siyum on Erev Pesach and do not fast. If the child who is a Bechor goes to a Siyum would the father still have to fast?
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in the case mentioned above i would believe that the father would still have to fast because there was no fasting at all done, since the son didn't need to fast therefore when he went to a siyum he ate, but he was allowed to eat anyway (i.e. it's not considered the son having to fast and then breaking it), i do think that it's possible that the son would patter the father if he actually fasted vtz"i
The father is expected to fast on behalf of his underage (ie., younger than 13) son who is not obligated to fast himself. If his son is underage but old enough to fast (eg., 12 and a half, for the sake of argument), I think the point is that the father ought to attend the Siyyum with his son, in order to obviate the need to fast. The son does not have to fast at 12.5 years old any more than he has to fast at that age on any other fast day. For Hinuch it's probably a good idea, but it's not necessary.
As for technical Halachah, I assume - without sources handy - that the father is still the one responsible for either fasting or attending a Siyyum. However, since this part is Minhag (although there is an opinion that the head of household is included in the fast regardless of age of children or personal status as a Bechor or non-Bechor), it may not be a matter for technical Halachah.