First, to answer your question: lots of good ideas already posted! Of course, change the baby. The bathroom is the only option, really.... and pull-ups are a GREAT idea for any age. They rip off the sides for disposal, and the child steps into the fresh one. Absolutely no need to lay the toddler down to take care of business.
Some other tips for travel-preparedness:
Dress your little girls in a skirt, dress a boy in elastic waisted pants (track pants, sweatpants) - - something EASY to pull up/down, on/off.
Use pull-ups, not diapers (see above)
Be prepared! Some kids have a pattern - - if you know they "go" 30 mins. after a meal, then feed them an hour before the flight if you can!
Good luck!