You are asking a very interesting question. There is no clear-cut answer for who exactly these Bnei Kohanim Gedolim were, but it is important to understand the social situation in the times the Mishnah is referring to.

As well known (Josephus mentions this), the last period of the second Beis ha'Mikdash was characterized by the tension between the Chachamim (the Perushim) and the Tzedukim (there was a third group called Isiyim, the Essenes, too). The Kohanim Gedolim at the time were appointed every other year by the Romans and, usually, the role was given to the highest bidder. Unfortunately, it seems that the families usually were on the Tzedukim side. In our Mishnah, we can see that the head of the Perushim, Raban Yochanan ben Zakai, preferred Chanan's opinion. On the other hand, Rebbi Dosa who was one of the oldest and honored Tana'im in his times, did agree to their opinion.

Some researchers think that the title "Bnei Kohanim Gedolim" were the Kohanim Gedolim who were removed from office for political reasons, and their families. This fits more with the "Haskamah" they got from Rebbi Dosa.

I hope this helps a bit,

Kol Tuv,

Aharon Steiner