Dear Rabbi Kornfeld,
At the bottom of daf 20b, we find a story of Ameimar, who tore Keri'ah three times. The events according to Tosefos, d'h "Atah" are; Ameimar tore Keri'ah while sitting down, then his son came in and tore Keri'ah while he was standing up, demonstrating for his father that to be "yotze" the Keri'ah, it must be done standing. Ameimar, having seen his son tear Keri'ah while standing was reminded that Keri'ah must indeed be done while standing, so he stood and tore Keri'ah a third time.
I remember a gemara in Kiddushin, but I can't remember where in Kiddushin the following advice appears: when a son sees his father doing something incorrectly, he shouldn't verbally correct him, but rather the son should find an opportunity to do the act correctly, thereby "reminding" his father of the correct way for the act. This seems to bear out the way Tosefos learns these events of Ameimar. Can you "track down" this gemara in Kiddushin for me?
The Gemara that you mention is in Kidushin 32a, where it says, "If one's father was transgressing a Mitzvah of the Torah, one should not say to him, 'Father, you are transgressing a Mitzvah of the Torah.' Rather, he should say, 'Father, there is a verse in the Torah that says such and such'," and he quotes for his father the verse so that his father will understand on his own accord that he made a mistake (RASHI there). You are correct that this seems to be the basis for the conduct of Ameimar's son, according to Tosfos.
All the best!
Mordecai