In the merit of the children
ויקרא אל-משה וידבר יקוק אליו מאהל מועד לאמר
[Hashem] called to out to Moshe; Hashem spoke to him from the Tent of Meeting, saying
The first verse in the book of Leviticus tells us that Hashem called out to Moshe from the Tent of meeting. He was going to teach him the laws of the Temple offerings. Hashem’s voice emanated from between the Keruvim, the Angel-like statues on top of the Aron HaKodesh, the Holy Ark. Only Moshe could hear the voice of Hashem, and only until the entrance of the Tent of Meeting. The verse which introduces this idea is written unusually: the א of ויקרא is written small, making it look like the word ויקר. This implies Hashem happened upon Moshe; it indicates a lack of intent. There are many suggestions given as to why the Torah wrote this word this way. The following is a unique approach.
There’s a Midrash which emphasizes just how special children are to Hashem. When the Sanhedrin, the Jewish Supreme Court was exiled from Israel, the Shechinah, the Divine Presence, didn’t follow suit. When the shifts of Kohanim and Leviim in the Temple were exiled, the Shechinah didn’t follow suit. However, when the children were exiled, Hashem (so to speak) exiled Himself with them. This demonstrates a special relationship between Hashem’s presence in this world and children. That is, specifically schoolchildren whose words of Torah are inherently free of sin. Their merit is what causes Hashem to dwell in this world.
According to this there is a better understanding of the essence of the Keruvim. As mentioned, between the two Keruvim is where Hashem’s voice emanated from. All of His commandments to the Jews came from there, to Moshe’s ear. What does the word Keruvim mean? The gemarra explains that it comes from the word כרביא, which is Aramaic for the likeness of a child. This teaches us that the golden statues of the Keruvim had the faces of children. This hints to the fact that only in the merit of children did Hashem contract His presence, to teach to His nation the Torah and the mitzvos.
We even see that the children were the primary force behind the Jews receiving the Torah. When Hashem wanted to give the Torah, He first asked for a guarantor that the Jews will observe the Torah. The Jews responded that the Avos, the Patriarchs are their guarantors. Hashem said that they are already obligated to him; they have themselves to worry about. It would be like lending money to someone, and using someone else as a cosigner who already is indebted to the lender. Hashem said to present a guarantor who is free of obligation. Who would that be? The Jews presented their children. Hashem asked the children if they agree to guarantee that their parents observe the Torah, and the children responded affirmatively. Hashem gave the Jews the Torah. We see from here that the children were instrumental in the Jews receiving the Torah at Mount Sinai. This is addition to the fact that the Shechinah’s resting amongst the Jews was due to the merits of the schoolchildren.
Therefore, at the beginning of the book of Leviticus, with the first “calling out” to Moshe from the two Keruvim, there is a hint with the small א in ויקרא. The letter אלף, has the same etymology as from the verse ואאלפך חכמה, I will teach you wisdom. That is, in the merit and power from the learning of the small ones, the schoolchildren, the Jews merited to have the Shechinah descend from the upper Heavens to dwell amongst them. In their merits the Jews received the Torah. This is why the first “calling out” to Moshe, the first command from the Tent of Meeting, hints to the learning of the children. It was only in their merit that this “calling out” occurred. The voice emanated between the two Keruvim, who resembled children. They’re the reason the Shechinah was in the Temple, and the reason why the Jews received the Torah.
 Based on Be’er Yosef to Leviticus 1:1
 Leviticus loc. cit.
 Exodus 22:25
 Rashi ad. loc.
 For example, see Kli Yakar ad. loc.
 Eichah Rabbah 1:32
 Eitz Yosef and Mattanos Kehunah ad. loc.’s explanation of משמרות. Cf. Maharzu ad. loc.
 It’s not clear where the Be’er Yosef saw to make this jump. Seemingly, it’s the only thing that makes children unique compared to adults
 Cf. Shabbos 119b, brought in Rambam’s Hilchos Talmud Torah 1:3
 Sukkah 5b; Chagigah 13b
 Midrash Tehillim 8:3, based on Psalms 8:3
 חייבים הם לי. It is not clear to me if this means they’re already obligated in the Torah, or they already owe Hashem something
 The Midrash says they borrowed nursing children from their mothers, and brought forth pregnant women. The women’s stomachs became transparent and the fetus became visible.
 Job 33:33
 The Be’er Yosef doesn’t mention this, but it works nicely that there is an old custom to have this parsha be the first verses of Torah that a child learns. This is because just as offerings are pure, children are pure. See Tanchumah parshas Tzav § 14; Vayikra Rabbah 7:3; Avos DeRabbi Nosson 6:2; Sefer Chasidim § 1140; Kol Bo § 74; Shach to Yoreh Deah § 245 s.k. 8; Be’er Heitev ad. loc. s.k. 8; Kitzur Shulchan Aruch 165:10; Aruch HaShulchan 245:11. See Tosefes Bracha from the Torah Temimah who apologizes that this minhag has become annulled. These sources were gathered from Nitei Gavriel Tiglachas Hayeladim VeHilchasam 24:1 fn. 1. See there where he brings that the Yefeh Toar to Bereishis Rabbah 51:9 says that in his day it wasn’t the custom to learn the beginning of Leviticus with one’s children. Nitei Gavriel also brings Torah Sheleimah Bereishis 2 § 274, who brings a Midrash which suggests starting to teach one’s child from Genesis. The Torah Sheleimah brings an explanation that when the Temple existed, the custom was to start from Genesis, as it’s filled with praises of Hashem. Once the Temple was destroyed, it was instituted to start from Leviticus, as reading about the offerings is considered as if they were brought in the Temple (see Vayikra Rabbah ad. loc. which says this is one of the reasons for this custom)