I would say it depends on the age.

5-11: shuffle. This is about the most minimal investment one can make, and at that age, the possibility of losing it is fairly high. Plus, fewer buttons and easier to navigate for small hands. Get it inscribed with their name/phone # just in case (free when you order at the Apple Store).

12+: Shuffle or Nano, depending on the kid. If kid is still losing/breaking things, shuffle. If they are able to take care of it (these are more delicate with the LCD etc), nano. I think it's also beneficial to get these inscribed. There some aftermarket lanyards available, including one by Apple that has built-in earbuds that would help to keep it attached to the kid.

If you want to keep them entertained for long periods, you might consider a Video iPod. You could then load up lots of cartoons/kids movies on to it for the flight. I think this would be too much to spend for a kid, but perhaps you could get it for yourself and have them use it for the trip. If you have multiple kids to entertain, you could get a headphone splitter so that they could both watch.

You'd probably also want to get a case to protect the ipod(s) from falls, contents of bags/pockets/etc. To find which accessories would be best for you, reviews of all things iPod are located at http://www.ilounge.com.

Have a good trip!