There is a product called EarPlanes, and are available in adult and child size. Most readily available at Wal-Mart, CVS, Rite-Aide, etc. A little expensive (around $4.00/ea.), but well worth it. Just be sure to read the instructions carefully, and don't make the mistake of jamming them in too far in your ears. . . the instructions state this. It is really a nice product, and can be a great benefit if used correctly. We use them every time we fly.
As a side note, and you will want to check with your pediatrician on this, but an antihistimine such as Benedryl before the flight does wonders, especially if you have a cold. Really opens you up so the flight does not make it worse. We have done this in the past also with much success.
Hope this helps, and have fun!