I don’t understand. The Rambam in Peirush haMishnayos in Chulin writes that we don’t observe milah because Avraham did milah, we don’t observe gid h’anasheh because Ya’akov did not eat gid ha’naseh. The only reason we observe mitzvos is because they were given at Sinai. How can a description of Avraham’s behavior serve as the source for the mitzvah of chinuch? Where is the mitzvah given at Sinai? (Hat tip to Doleh U’mashkeh’s post here that made me think of this.)
2) Our parsha gives us an insight into how to do chinuch. Avraham welcomed guests into his home and rushed to prepare a meal for them. Instead of shechting the cows himself, he delegated the job to Yishmael because Avraham wanted to be mechanech Yishmael in mitzvos.
Chazal tell us whatever Avraham personally did for his guests, Bnei Yisrael received in return directly from Hashem when they needed food and water in the midbar. Whatever Avraham delegated to others, Hashem gave to Bnei Yisrael indirectly as well. Why should Bnei Yisrael have gotten a less than ideal reward just because Avraham delegated a job to Yishmael -- he deliberately did it that way for the sake of the mitzvah of chinuch?!
Many answer that chinuch is not done by delegating, by telling a child what to do. Chinuch is done by setting an example. Yishmael witnessing Avraham personally tending to his guests would impress the lesson of chessed upon him far more than Avraham telling him to shecht the cows on the guests behalf.