In the first dream, the 12 brothers bundles sheaves in the field represents the gathering of the 12 Tribes together. The bundles of the Tribes turned towards Yosef, like the people gathering around the king to hear the words of the Torah. Finally, the bundles of the tribes bowed down, representing the Bittul and Fear of Hashem which Hakhel is meant to inspire.
The Torah specifies that Hakhel is a gathering for men, women and children. This is alluded to in the second dream of Yosef where the sun, moon and 11 stars bowed down. The sun, as a giver of light (Mashpia), represents the men. The moon, as a receiver of light (Mekabel) represents the women and the stars represent the children.