At nine months, and with three adults the trip should be easy, I think!! Assuming you have no seats reserved for the babies and will be having them on your laps on the plane, here's what I would do:
- Check as much luggage as possible, both carseats and the stroller at the ticketing area.
- For this age, I would recommend using a pouch for the plane, but you might want a regular back or front carrier like a meitai or ergo for walking through the airport. The ergo will be easier to get on and off for passing through security - the carrier will have to go through the xray machine, while both babies will have to be carried through the checkpoint. This is a challenging point if you are traveling alone with twin babies, but you should be fine, given that there will be three of you. Back to the pouch, I found this type of carrier great for the plane. The babies were comfortably contained while sitting on my lap during the flight, and it was nice to have them swaddled in this way that left my hands free. I really missed it this time when we flew - both of my boys are too big to cradle in the pouch now!
- Plan to carry on one or two diaper bags. Include enough diapers that if they each poop every hour of the trip, you have enough. I know this might seem excessive, but I definitely travel in the "better safe than sorry" camp, especially after our experience in December of being stuck at the airport over night.
- If you are not traveling with the nursing mama, be sure to bring double the amount of EBM or formula than you will need. I like to travel with Playtex bottles - one bottle for each baby, and plenty of liners and clean nipples so I can swap them out easily.
- Aside from the diapers and feeding supplies (if needed), I'd just bring a handful of the favorite toys and a couple of books for this age. Novelty can be important later, but I think at 9 months, favorites would be enough. When in doubt, though, bring something new. Tie long strings onto each toy so that when they fall on the floor, you'll be able to pull them up by the cord. Traveling with 1 year olds, I found the single most important item was a portable DVD player. Without it on our last trip, it would have been pretty unbearable. My boys didn't pay any attention to TV until very recently, though, so again, I think you'll be fine with just toys.
- Although I couldn't get my hands on any for the last flight, a friend and naturopath recommended getting some Herbs for Kids Super Calm. I tried using melissa and valerian alone on the last flight and didn't notice any really helpful effect. I did call my pediatrician about using benadryl, and they strongly discourage it. The APA does not approve the use of benadryl in this situation, according to my physician's office.
- Finally, know that legally the airline cannot allow two lap babies in the same row. If you got three adult tickets seated together, they will move one of you with a baby. Pack your diaper/food/toy bag accordingly, because if you don't change your seat assignment early, you might be sitting quite far away from each other. This happened to us this time on the way to MA - I was flying with my mom, and we were split up.
I hope this helps! Have fun, and let us know how it went, along with any new tips you might glean from your trip!