The gemara raises a question whether the heter of a worker to eat as he works is: משלו הוא אוכל או משל שמים הוא אוכל . Is he eating from his own which means it is like added benefits in addition to his salary that he has full rights over to give to someone else, or is he eating from shamayim which rashi explains to mean is like a gift of gemilas chessed that the employer is required to give him by the torah? The gemara has a long discussion and concludes that it is a machlokes.
There seems to be a dispute between rashi and tosafos in defining the tzad of משל שמים הוא אוכל. Rashi explains that he isn't entitled to it until it "reaches his hand" so that he can't stipulate that it should be given to his children. Rashi implies that if the worker himself would take the grapes and hand it to his son who is walking next to him (but not a worker), that would be permitted even according to this tzad. Tosafos disagrees with rashi (pointed out by GR"A) and says that the tzad of shamayim holds that he is only zocheh in it at the moment he places it into his mouth and therefore has no way to give it to his son.
On 92b, when the gemara quotes the mishna that one cannot stipulate with the employer that his child (katan) won't eat, and instead the employer should compensate for it in the salary. The reason that on the tzad of משל שמים הוא אוכל we fully understand why the father can't impose this stipulation on his child, rashi explains -
דאין לפועל קטן זכות אלא כשנותן לתוך פיו
This seems to contradict rashi on amud alef who says that even on the tzad of shamayim, the worker is zocheh as soon as it reaches his hand, even before he puts it in his mouth?
The answer is that Rashi holds that an adult worker in general has an ability to make a kinyan and therefore acquires the fruit that he is entitled to eat as soon as he picks it up (even on the tzad of mi'shel shamayim). But a child who doesn't have the ability to make a kinyan cannot be zocheh upon picking it up, and is only zocheh at the time he actually eats it. Therefore, the father cannot stipulate that the employer should increase the salary in exchange for the father giving up the childs right to eat, because the right to eat only exists when he is actually eating.