Just to clarify a bit as a follow-up on yesterday's post:
The problem with learning that the Rambam understood that the mitzvah d'oraysa of "yeira'eh kol zechurcha" includes bringing one's children to the Mikdash is that the Rambam himself writes in Hil Chagiga 2:3 that bringing children is "kdei lchancho b'mitzvos" -- a din in chinuch.
You could say the Rambam in Mishne Torah recanted on his position in Sefer Hamitzvos if you like, but that is a bit of a dochak.
To be fair, the Rambam in Mishne Torah continues and adds after "kdei lchancho... she'ne'emar yeira'eh kol zechurcha." So we have to fudge something here-- either the "kdei lchancho" is lav davka or the pasuk is lav davka.
The tzad to say the quote of the pasuk is a lav davka is the fact that the gemara 4a quotes a braysa that uses the same pasuk to explain why a katan is chayav but then reinterprets the braysa to be an asmachta. The Rambam is simply echoing the braysa's language with the gemara's interpretation (Lechem Mishne).
My son pointed out a piece from R Dovid Soloveitchik in which he argues that this cannot be a regular din of chinuch because the katan does only half the mitzvah -- he comes to the Mikdash, but does not have to offer korbanos (see the meforshei haRambam who point out that this is a machlokes Rashi and Tos). How can you have chinuch to do half a mitzvah? He therefore concludes that the words "kdei lchancho" cannot be taken at face value and ketanim come so that their father fulfills the d'oraysa din of "yeira'eh kol zechurcha." Note to that the Rambam does not just say "chayav l'ha'aloso" but adds "l'haros bo," implying a chiyuv on the father to appear with his children.
Sof kol sof: no simple conclusion here.