Reference for Arakhin 2:6
אֵין פּוֹחֲתִין מִשְּׁנֵים עָשָׂר לְוִיִּם עוֹמְדִים עַל הַדּוּכָן, וּמוֹסִיפִין עַד לְעוֹלָם. אֵין קָטָן נִכְנָס לָעֲזָרָה לַעֲבוֹדָה אֶלָּא בְשָׁעָה שֶׁהַלְוִיִּם עוֹמְדִים בַּשִּׁיר. וְלֹא הָיוּ אוֹמְרִים בְּנֵבֶל וְכִנּוֹר אֶלָּא בַפֶּה, כְּדֵי לִתֵּן תְּבַל בַּנְּעִימָה. רַבִּי אֱלִיעֶזֶר בֶּן יַעֲקֹב אוֹמֵר, אֵין עוֹלִין לַמִּנְיָן, וְאֵין עוֹמְדִים עַל הַדּוּכָן, אֶלָּא בָאָרֶץ הָיוּ עוֹמְדִין, וְרָאשֵׁיהֶן מִבֵּין רַגְלֵי הַלְוִיִּם, וְצוֹעֲרֵי הַלְוִיִּם הָיוּ נִקְרָאִין:
There were never less than twelve Levites standing on the platform and their number could be increased indefinitely. No minor could enter the [Temple] courtyard to take part in the service except when the Levites stood up to sing. Nor did [the minors] join in the singing with harp and lyre, but with the mouth alone, to add flavor to the music. Rabbi Eliezer ben Jacob said: they did not count in the required number, nor did they stand on the platform. Rather they would stand on the ground, so that their heads were between the feet of the Levites. And they were called "the youth of the Levites".