Just my two cents worth, my mom always did an awesome job having stuff for us to do when travelling as little kids. Our family favourite (to this day!) is audiobooks; there are so many options geared for kids that can be played in the car or put onto a phone/iPod for individual listeners. Hearing a story could distract us for hours. Probably won't work with the youngest child, but the older child might be quiet for an hour or two.
She also packed things like a deck of Uno! cards, and magnetized travel games (like checkers, with magnets keeping the pieces stuck to the board so that they weren't easily lost during bumpy stretches).