Why can't a minor's rights to eat while harvesting crops be waived?

Question

The Torah grants workers who are harvesting a field to consume some of it while on the job.

The Mishna in Bava Metzia (7:6) points out that a worker can waive his Biblically-granted rights, and make a deal that he won't eat any of the produce. He can similarly make an agreement on behalf of his workers, or his adult children. However, the Biblical rights cannot be waived for a minor (or for an animal, for that matter).

ז,ו קוצץ אדם על ידי עצמו, ועל ידי בנו ובתו הגדולים, ועל ידי עבדו ושפחתו הגדולים, ועל ידי אשתו--מפני שיש בהן דעת. אבל אינו קוצץ לא על ידי בנו ובתו הקטנים, ולא על ידי עבדו ושפחתו הקטנים, ולא על ידי בהמתו--מפני שאין בהן דעת.

Is the reason for this:

To waive one's Biblical rights (in this case? Generally?) is an act that requires Halachic capacity for consciousness -- da'at

Or is it:

It is too much to ask of the self-control of a child to pick berries but not eat a few.

Answer 1

It looks like it might be a little of both, actually. Consent (daas) is necessary for this to work, even with adult children or other workers (Shulchan Aruch, Choshen Mishpat 337:16 and Sma :37), and as you said, children are unable to legally grant such consent. But the Gemara (Bava Metzia 92b) also mentions the consideration that "Hashem has not granted [the father] rights to cause distress to his minor son or daughter" - i.e., your second point, that it would be too stressful for them to not be able to eat some of the food they're working with.