Mishnayos Bava Metzia Perek 7 Mishnah 6

A man can stipulate on his own behalf that he receive a certain increase in his wages instead of eating the produce with which he works, and similarly, he can stipulate this on behalf of his adult son or daughter, on behalf of his adult Canaanite slave or Canaanite maidservant, or on behalf of his wife, with their agreement, because they have the basic level of mental competence, i.e., they are legally competent and can therefore waive their rights. But he cannot stipulate this on behalf of his minor son or daughter, nor on behalf of his minor Canaanite slave or Canaanite maidservant, nor on behalf of his animal, as they do not have the basic level of mental competence.

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: