Pack a bag with books, snacks, extra water, gum for ear popping, an emergency kit, extra clothing layers, small games and playing cards, smartphones, tablets and headphones.
Austin Adventures' personal favorite: Pack a surprise plane bag that you don't let them see until they board. Include cheap trinkets like a coloring book, action figure, book, crayons, little sets of Legos, and/or a few favorite snacks/sweets that they will discover on the flight and keep themselves entertained for hours.