Dinei derabban and Divrei Sofrim (v)

Final thought on REW's chiddush:
The Mishna in Megillah (2:4) brings a machlokes Tana Kamma and R’ Yehudah whether a katan may read megillah. R’ Yehudah says a kattan may read because although he is mechuyav only m’derabbanan because of the mitzvah of chinuch, everyone else also shares the same level of chiyuv only m’derabbanan. The Rishonim debate why the Tana Kamma disagrees. Ramban writes that according to Tana Kamma the katan has no chiyuv at all to read; the chiyuv of chinuch is on his father to teach him. Tosfos explains that the katan has 2 derabbanans: his age, and the actual obligation of keriya; everyone else has only one derabbanan, therefore the chiyuvim are not parallel. It would seem that there is a fundamental issur at stake here: does the mitzvah derabbanan of chinuch actually obligate a child in mitzvos, or is it just a chiyuv on his father to teach? REW asks, if a katan has no chiyuv in any d’oraysa, what can possibly bind him to observe the mitzvah derabbanan of chinuch? It must be that the ratzon Hashem of the derabbanan even sans tzivuy creates an obligation. This ra’aya also stikes me as weak and almost paradoxical. The only way we know a katan is indeed patur from any mitzvos is because Chazal have told us so – if one rejects the katan’s obligation to adhere to Chazal because he/she is patur from lo tasur, then one must reject the ptur from all mitzvos that Chazal established through their limud. One is tempted to suggest that a katan is really chayav in mitzvos and his status as minor is just a ptur onshim, but that would seem to assume that chinuch is a din d’oraysa and I am not sure one can explain why a katan is not included in arvus.
I would like to return to the machlokes Rambam and Ramban that seemed to propel R' Elchahan into this chiddush. These mystical ideas of listening to ratzon Hashem without a concrete din or tzivuy are not something I think a Brisker would be happy with, and R' Chaim indeed had an alternative explanation. To be continued...