I like the lomdus behind the question: does the Rama mean the chiyuv ta’anis is on the katan, but since the katan can’t physically fast the father is mekayeim the ta’anis on his behalf, in which case the katan can make the siyum, or does the Rama mean that the chiyuv ta’anis is on the father, not on the katan, in which case the katan’s siyum doesn’t help?
R’ Chaim seems to assume as a davar pashut that the siyum of a katan is enough of a simcha to override the fast. I couldn’t find it last night, but I seem to recall that the Rogatchover raises this as a question. If the idea of a siyum is to celebrate completing a mitzvah, maybe the siyum of a katan who has no chiyuv in mitzvos doesn't count. Can a woman make a siyum (whether women have to fast ta'anis bechorim seems to depend on stiros in Midrashim) to remove the chiyuv of fasting?
2) Someone came to R’ Chaim Kanievsky and said that he decided to fast instead of doing the usual siyum for ta’anis bechorim. R’ Chaim paskened that since he made a siyum in past years, he needs to do a hataras nedarim to break the minhag.