Reference 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."

Jastrow