Rashi (3:1) writes that Aharon's children count as "toldos Aharon **u'Moshe**" because since Moshe taught them Torah it is as if he was their father.
The meforshei Rashi all ask: Moshe taught Torah to all of Klal Yisrael. What was special about his teaching Torah to Aharon's children that created a father-son relationship?
Maharal in Gur Aryeh answers that without Klal Yisrael to teach to, Torah would never have been given to Moshe. Moshe was obligated to teach them. However, what Moshe taught to bnei Aharon was above and beyond what was demanded of him.
What does Maharal mean? There is no shiur to the mitzvah of talmud torah saying you have to teach others up to point X and no more. There is no point which is above and beyond one's obligation. So how could what Moshe taught bnei Aharon be above and beyond what was required?
See R Gifter's answer in his Pirkei Torah.
I would suggest as follows: last night I posted R Wahrman's chidush that for something to be defined as part of the cheftza shel torah it is not enough for Moshe to have received it from Hashem -- it also has to be passed on to others. A kabbalas hatorah without mesorah of Torah is by definition incomplete.
The chovas hagavra of what one is obligated to teach others has no bounds. However, perhaps not every detail need be taught before one can say a halacha is part of the cheftza shel torah. As an analogy, it's fair that say someone who knows shulchan aruch with Shach and Taz etc knows basar bchalav, but that doesn't mean they are exempt from continuing to learn those sugyos, as Torah is endless. What Maharal may mean is that Moshe had to transmit some basic level of Torah to all of Klal Yisrael for there to be a cheftza shel Torah. However, when it came to the chovas hagavra of limud Torah, the endless depths of learning, he devoted his energies in particular to teaching Aharon's children.