For many generations there has been a custom to begin a child’s learning of Chumash with Sefer Vayikra, which Chazal call Torat Kohanim. The explanation is found as far back as the midrash (Yalkut Shimoni, Tzav 479): "Why do young children start with Torat Kohanim? Let them start with Bereishit? Since the korbanot (sacrifices) are pure and the children are pure, let the pure come and deal with the pure."
Vayikra is not a book whose purpose is just to provide practical instructions on the way to bring sacrifices. Rather, it deals, on a fundamental basis, with Bnei Yisrael as a mamlechet kohanim v’goy kadosh (kingdom of priests and a holy nation). There is no more appropriate time to inculcate these values into children as when they have the freshness and purity of young age.
The bringing of korbanot is the essence of avoda (service of Hashem), which along with Torah and gemilut chasadim (acts of kindness) are the pillars that keep the world standing (Avot 1:2). The Torah represents the thought-related element of Judaism; gemilut chasadim is the active part between man and his fellow man. However, these two are insufficient without avoda, the active part of our proper connection with Hashem, which also must exist in order that the proper behavior between man and man will have its full meaning. We need to use the hand (action) and the heart (thought) in making our relationship with Hashem complete. The avoda must come from within a person, as korbanot should not be offered as some sort of external donation but as a gift from one’s essence The prophets (see Yeshaya 43:23, for one example) spoke very strongly against the phenomenon of people offering korbanot without the correct frame of mind or actions, which Hashem said He has no interest in.
On the other hand, we must reject that which some say that since the main thing is what is in a person’s heart, it is enough to serve Hashem with one’s heart. This reminds us of the gemara (Yevamot 109b) that says that whoever says that he has only Torah does not even have Torah. The heart does not have real value if it is in a manner that is disconnected from action.
Rabbi Shimon Bar Yochai said that the pasuk, "This is the Torah of the olah sacrifice" refers to the atonement for the thoughts to sin (Vayikra Rabba 7:3). This is because the heart itself needs protection. Therefore, actions are needed to protect the heart from going into morally dangerous thoughts. On one hand, the korbanot are given as if from our very essence, by means of the thought process. In practice, though, they are brought from the cattle and the flock of sheep.
This matter of avoda has to be learned well and from an expert teacher. That is why Moshe, who thought he had finished his leadership role after the Exodus and the giving of the Torah, was told that he had a greater role still ahead of him: to teach Israel the laws of purity and of korbanot (Tanchuma, Vayikra 4).