The first day the food truck is active, and they're handing out free sandwiches to the workers who helped them load the heavy gear onto the truck. One of the sandwiches is burned, and Carl's son Percy doesn't understand why they don't give it out anyway because its not like they're being paid for the food. Carl takes Percy aside and explains to him just what being a chef means, and why you don't give your customers anything but your best effort, always.
Carl: I may not do everything great in my life, but I'm good at this. I manage to touch people's lives with what I do and I want to share this with you. Understand? Percy:[a little awestruck] Yes, chef!
At the end of the first day that the food truck is open, Martin gives Percy the greatest compliment one professional cook can give to someone by acknowledging the ten-year-old as a colleague and an equal:
Martin: You're not ten. You're kitchen staff!
The scene where Carl buys Percy his first chef's knife. As any professional chef will tell you, your knives are YOUR knives, and as silly as it sounds, each chef builds up a relationship with his own set of tools. In trusting Percy with his own chef's knife, Carl is telling Percy that he is worthy of being counted not only as Carl's son, but as a trusted colleague in the kitchen.
Percy's video, a montage of one-second clips that covers the course of the food truck road trip. After finally watching it, Carl is inspired to let Percy continue working in the truck even during the school year. Naturally, this is under the strict condition that he doesn't let it distract him from doing well in his studies.
The whole movie is built as such. Once Carl leaves the restaurant and decides to get a food truck, the final hour basically maps out the happiness that can emerge from throwing caution to the wind to do something you truly love the way you want to do it.
Just Martin's presence as one of El Jefe's three-man staff. He's both a great friend to Carl, and an uncle to Percy. It's clear that Carl would have had some trouble if it were only the two of them, or it was just him and he had to hire unfamiliar people as staff.