Games. They're what childhood is all about, and they'll keep a kid happy and quiet even more reliably than branded superhero snack food.
You'll probably want to plan ahead here so you're not caught drawing a blank with all the stress of travel. Make a list of low-noise games you can play on the plane or in the car. There are tons of them -- get a point for spotting red cars or drivers wearing glasses; guess the names of other passengers; bring a deck of cards, a mini checker board, a Mad Libs book or some paper to play Pictionary.
There's also the classic Punch Buggy (aka Slug-a-Bug) for long car rides, but that does require some restraint on the part of the punchers, so use your discretion.
Up next: "Quiet" doesn't have to mean silence…