Bearing in mind the Pasuk in Vayikra (8:7 - 13), which indicates that Moshe first dressed Aharon the belt and then his sons, why does the Torah here write "v'Chagarta osam Avnet, Aharon u'Vanav", implying that he girded them all the belt, one after the other?
Ramban: The Torah incorporates Aharon with his children because it did not mention the belt earlier in Pasuk 5, 6. 1
Rashbam #1: This gives rise to the Machlokes as to whether the belt of the Kohen Hedyot contained Sha'atnez, like that of the Kohen Gadol, or not. 2
Rashbam #2: According to the simple P'shat, this is the way the Torah speaks, and one need not take the change too seriously.
Precisely because Aharon wore a belt just like they did. The Torah mentioned the Mitznefes however, because it differed from the Migba'os that his sons wore (Ramban [Refer to 28:4:4:2). The question arises however, why the Torah mentioned the shirt by Aharon (See Pasuk 5) - seeing as he wore the same shirt as his sons?
Why does the Torah not mention the underpants here?
Ramban (above, Shemos 28:3): Because it mentioned earlier 1 that a Kohen who performs the Avodah without the underpants is Chayav Misah, it is obvious that they will wear them and it is unnecessary to repeat them here. Moreover, the Torah separates the underpants from the other Begadim because whereas Moshe dressed them in the other Begadim, 2 the underpants, which were to cover their Makom Ervah, they themselves put them on due to Tzeni'us.