Kitzur Tefilat Amecha #38-39

[adapted for middle-schoolers by Rabbi Shmuel Jablon from Rav Aviner's three-volume commentary on the siddur "Tefilat Amecha"]

We have all been saying Kriyat Shema for many years. So we all know the words well. But we need to make sure we really understand what’s happening in Shema. When we say the first verse and then the first parashah of Shema, we are accepting the Yoke of the Heavenly Kingdom. We are saying that we believe in Hashem, love Hashem, and want to be close to Hashem. And that’s really the first thing we need to accomplish. But then we go further in the second paragraph. There we are accepting the Yoke of the Mitzvot. Once we say that we believe in Hashem, we are recognizing that Hashem has things He expects us to do - His Mitzvot. Belief isn’t enough, it must bring us to observance and action.

We say Shema - which is the center of Judaism. We proclaim that Hashem is Our G-d and that He is One. Not only is He the Only G-d, all of the powers that exist in the world come from Him and are ruled by Him. We then quietly say a verse that is not in the Torah - Blessed is His Honored Name, His Kingship is Forever. This shows we don't only accept Hashem in theory, we accept him in fact in our world and forever.
The Midrash teaches us that when Yaakov Avinu knew he was dying, he asked his sons if they would follow Hashem. They answered with the verse Shema Yisrael (Listen, Yisrael – Yaakov's name). Yaakov Avinu was so relieved and happy that his children would follow belief in Hashem that he quietly said "Baruch Shem." Therefore, we say this quietly, too. We also accept Hashem as our G-d.
Hashem wants us to accept Him as our King. It brings Hashem joy to know that we, His children, accept Him as our King. So we need to say "Baruch Shem." But we say it quietly as He doesn't need our acceptance to be King of the Universe. So it is not on the same level as Shema Yisrael, the actual verse in the Torah. Still, it is all part of our stating clearly and forever that we accept Hashem's Kingship.