Responsa for Bava Metzia 7:6
קוֹצֵץ אָדָם עַל יְדֵי עַצְמוֹ, עַל יְדֵי בְנוֹ וּבִתּוֹ הַגְּדוֹלִים, עַל יְדֵי עַבְדּוֹ וְשִׁפְחָתוֹ הַגְּדוֹלִים, עַל יְדֵי אִשְׁתּוֹ, מִפְּנֵי שֶׁיֵּשׁ בָּהֶן דָּעַת. אֲבָל אֵינוֹ קוֹצֵץ עַל יְדֵי בְנוֹ וּבִתּוֹ הַקְּטַנִּים, וְלֹא עַל יְדֵי עַבְדּוֹ וְשִׁפְחָתוֹ הַקְּטַנִּים, וְלֹא עַל יְדֵי בְהֶמְתּוֹ, מִפְּנֵי שֶׁאֵין בָּהֶן דָּעַת:
One (i.e., a worker) may stipulate [(to take money instead of eating] for himself, for his grown son and daughter, for his grown bondsman and bondswoman, and for his wife, because they have "knowledge," [and they know and waive it]. But he may not stipulate thus for his minor son and daughter, his minor bondsman and bondswoman, and his beast, for they lack "knowledge."
Shut min haShamayim
They responded: even though this is permitted, we only share this ruling with the pious, lest it is seen is public and the masses come to transgress.*The language of the question comes from a responsum of Rav Saadia Gaon, who forbids this practice of asking a non-Jew to buy on Shabbat. The responsum is quoted by the Rosh on Bava Metzia 7:6:1, and although Saadia's reasoning is rejected, it seems that he agrees with the ruling - although maybe the ambiguity reflects something of the answer given here.
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