Pack anything fits in a carry-on bag that will help keep your child entertained for as long as possible. That means toys, coloring books, reading books, stuffed animals, dolls, interactive learning toys, DVD players, MP3 or other music players --whatever you have that works. If you can afford it, buy some new toys, books, DVDs or music just for the flight.
Don't bring them all out at once, though. Bring out one item at a time for your child to play with. Some parents wrap up new items and give them to their kids to open at different stages during the flight.