To add a further point: Regarding the intent motivating the mitzvos, there is more pressing relevance to the above conceptוּלְהוֹסִיף, דִּבְכַוָּנַת הַמִּצְוֹת, נוֹגֵעַ עִנְיָן זֶה
[i.e., that even the loftiest truths should be comprehended from the understanding of the ideas in their simple sense,][שֶׁעִנְיָנִים הֲכִי נַעֲלִים, יֵדְעוּ מֵעִנְיָנִים אֵלֶּה כְּמוֹ שֶׁהֵם בְּפַשְׁטוּת]
than regarding Torah study.עוֹד יוֹתֵר מֵאֲשֶׁר בַּתּוֹרָה.
For the positive virtue in “the wonders of the Torah” being comprehended from the simple meaning of the Torah’s teachingsכִּי הַיִּתְרוֹן בָּזֶה שֶׁיְּדִיעַת הַנִּפְלָאוֹת הִיא מֵהָעִנְיָנִים שֶׁבְּפַשְׁטוּת,
is that through this process, man’s knowledge of “the wonders of the Torah” will be internalized and will relate to his own understanding (as explained in sec. 4 above).הוּא שֶׁעַל-יְדֵי-זֶה תִּהְיֶה יְדִיעַת הַנִּפְלָאוֹת דְּהָאָדָם גַּם מִצַּד עִנְיָנוֹ (כַּנַּ"ל סְעִיף ד),
The purpose of mitzvos is to refine the created beings (as explained in sec. 5 above),וְכֵיוָן שֶׁעִנְיַן הַמִּצְוֹת הוּא לְצָרֵף בָּהֶן אֶת הַבְּרִיּוֹת (כַּנַּ"ל סְעִיף ה),
i.e., to bring about refinement in the person observing the mitzvahלִפְעוֹל בֵּרוּר וְזִכּוּךְ בְּהָאָדָם הַמְקַיֵּם אֶת הַמִּצְוָה,
[the fundamental dimension of that refinement is achieved through the intentions that motivate the mitzvos,1 for that affects a person’s mind and feelings;[דְּעִקַּר הַבֵּרוּר וְהַזִּכּוּךְ הוּא עַל-יְדֵי כַּוָּנַת הַמִּצְוֹת,
and the loftier the intention motivating the mitzvah, the more comprehensive the refinement it will bring about].וְכָל שֶׁהַכַּוָּנָה הִיא נַעֲלֵית יוֹתֵר, הַבֵּרוּר וְהַזִּכּוּךְ הוּא נַעֲלֶה יוֹתֵר],
Therefore, emphasizing that the comprehension of the “wonders,” i.e., the mystic dimensions of the intentions that motivate the mitzvos are to be perceived through the intentלָכֵן בְּעִנְיַן הַמִּצְוֹת נוֹגֵעַ עוֹד יוֹתֵר שֶׁיְּדִיעַת הַנִּפְלָאוֹת דְּכַוָּנַת הַמִּצְוֹת תִּהְיֶה מִתּוֹךְ הַכַּוָּנוֹת שֶׁבְּפַשְׁטוּת.
motivating the mitzvah as understood on a simple level, is even more pressingly relevant regarding mitzvos than the similar thrust with regard to Torah study. For the internalization of these mystic truths will enable the person’s refinement to be more complete and encompassing.
On this basis, it is possible to explain why, in the the maamar entitled Shoresh Mitzvas HaTefilah (“The Core of the Mitzvah of Prayer”),2 the Tzemach Tzedek cites the expression:וְעַל-פִּי-זֶה יֵשׁ לְבָאֵר מַה שֶּׁמֵּבִיא הַ'צֶמַח צֶדֶק' בַּמַּאֲמָר 'שֹׁרֶש מִצְוַת הַתְּפִלָּה' הַלָּשׁוֹן
“I pray with the intentions of a young child.”אֲנִי מִתְפַּלֵּל לְדַעַת זֶה הַתִּינוֹק.
On the surface, it is not appropriate to include that statement in the maamar, Shoresh Mitzvas HaTefilah. That maamar explains the concept of prayerדְּלִכְאוֹרָה, בַּמַּאֲמָר 'שֹׁרֶש מִצְוַת הַתְּפִלָּה' מְבָאֵר עִנְיַן הַתְּפִלָּה
[as a whole, as well as several particular aspects of prayer,][בִּכְלָל, וְגַם כַּמָּה עִנְיָנִים פְּרָטִיִּים דִּתְפִלָּה]
according to Kabbalah, and furthermore, according to Chassidus, which reflects an even loftier approach.3עַל-פִּי קַבָּלָה, וְעוֹד יוֹתֵר – עַל-פִּי חֲסִידוּת,
In such a maamar, how is it fitting to cite the statement, “I pray with the intentions of a young child”?וּמַה מָּקוֹם בְּמַאֲמָר זֶה לְהָבִיא הָעִנְיָן דְּ'אֲנִי מִתְפַּלֵּל לְדַעַת זֶה הַתִּינוֹק'.
It is possible to say that by doing so, the Tzemach Tzedek alludes to the concept that even the most sublime intentions of prayerוְיֵשׁ לוֹמַר, שֶׁבָּזֶה מְרַמֵּז, שֶׁהַכַּוָּנוֹת דִּתְפִלָּה בְּדַרְגוֹת הֲכִי נַעֲלוֹת,
must be connected to the concept of prayer as understood by a young child.צְרִיכוֹת לִהְיוֹת קְשׁוּרוֹת עִם עִנְיַן הַתְּפִלָּה כְּמוֹ שֶׁהִיא 'לְדַעַת הַתִּינוֹק',
This includes both the concept of prayer as a whole,הֵן בְּנוֹגֵעַ לִכְלָלוּת עִנְיַן הַתְּפִלָּה –
that the knowledge of the loftiest levels of prayer should be characterized by the approach to prayer of a young child, the artless call of the child to his Father in heaven,שֶׁיְּדִיעַת עִנְיַן הַתְּפִלָּה בְּדַרְגוֹת הֲכִי נַעֲלוֹת תִּהְיֶה גַּם מֵעִנְיַן הַתְּפִלָּה כְּמוֹ שֶׁתִּינוֹק מֵבִין,
and also, the explanation of the words (the verses, the requests, and the like) recited in prayer.וְהֵן בְּנוֹגֵעַ לְבֵאוּר הָעִנְיָנִים (הַפְּסוּקִים, בַּקָּשׁוֹת וְכוּ') שֶׁאוֹמְרִים בַּתְּפִלָּה,
Even the loftiest explanations of the prayers must be aligned with [and, indeed, the awareness of them should come from] the simple meaning of the words of prayerשֶׁהַבֵּאוּרִים הֲכִי נַעֲלִים צְרִיכִים לִהְיוֹת בְּהֶתְאֵם [וְעַד שֶׁיְּדִיעָתָם תִּהְיֶה מִתּוֹךְ] פֵּירוּשׁ הַמִּלּוֹת דִּתְפִלָּה
as understood by a young child.כְּמוֹ שֶׁתִּינוֹק מֵבִין.
Although this approach is relevant to all aspects of the Torah,וַהֲגַם שֶׁעִנְיָן זֶה הוּא בְּכָל הָעִנְיָנִים שֶׁבַּתּוֹרָה,
nevertheless, since prayer involves man’s ascent from below,מִכָּל-מָקוֹם כֵּיוָן שֶׁעִנְיַן הַתְּפִלָּה הִיא הַעֲלָאָה מִלְּמַטָּה לְמַעְלָה,
i.e., his personal refinement,בֵּרוּר וְזִכּוּךְ הָאָדָם,
therefore, this approach is more relevant with regard to prayerלָכֵן בַּתְּפִלָּה עִנְיָן זֶה נוֹגֵעַ יוֹתֵר
(in a way similar to the explanations given above regarding mitzvos).(עַל-דֶּרֶךְ שֶׁנִּתְבָּאֵר לְעֵיל בְּנוֹגֵעַ לַמִּצְוֹת).
Therefore, the maamar, Shoresh Mitzvas HaTefilah cites the statement, “I pray with the intentions of a young child.”וְלָכֵן, בַּמַּאֲמָר 'שֹׁרֶש מִצְוַת הַתְּפִלָּה' מוּבָא הָעִנְיָן דְּ'אֲנִי מִתְפַּלֵּל לְדַעַת זֶה הַתִּינוֹק'.
Towards the end of sec. 4, the Rebbe stated:
The starting point for everything that occurs in the world at large and in the individual world of every person is found in the Torah. Therefore, when (a person) sees the wonders of the Torah through the stories and laws in the revealed Torah – i.e., the body and the garment of the Torah reveal the soul and “the soul of the soul” of the Torah – this serves as a catalyst for a similar process to occur within the person who studies the Torah. He is able to perceive the wonders of the Torah (even) within the context of his own human circumstances.
A person should perceive that the different approaches to Torah study, pshat, remez, derush, and sod – i.e., all the levels ranging from the simple, down-to-earth understanding to the loftiest mystic truths – are not discrete disciplines but part of one organic whole. In sections 5-7, he explained one should view mitzvos with the same approach; indeed, such an approach is more relevant with regard to mitzvos, for mitzvos relate to man as he exists in his own context. On this basis, in the sections to come, the Rebbe explains how the difference between the spiritual and the physical that characterizes the world at large reflect only the world’s initial state. Ultimately, man’s Divine service will reveal how G‑d’s oneness pervades all existence and the spiritual and the physical are both integral dimensions of a single unity.

Summary

The concept that even the Torah’s loftiest spiritual truths should be understood from the simple understanding of the ideas is particularly relevant with regard to mitzvos because the purpose of mitzvos is to bring about refinement in the person observing the mitzvah.
On this basis, it is possible to explain why, in the the maamar entitled Shoresh Mitzvas HaTefilah, the Tzemach Tzedek cites the expression: “I pray with the intentions of a young child.” By doing so, the Tzemach Tzedek alludes to the concept that even the most sublime intentions of prayer must be connected to the concept of prayer as understood by a young child. Since prayer also involves man’s self-refinement, the perception of the integration between the spiritual meaning and intent of the prayers and their simple meaning is particularly relevant.