The bigger they get the more they can help you carry all their stuff!
This list is assuming you are also checking luggage. Our method for long flights (I'm talking 25-30 hours travel time with 16 hour legs) is to have 2 diaper bags. One bag goes at our feet and one up in the overhead bins. Each is complete with some of all the items, and the bag at our feet can be re-stocked during layovers or mid-flight when depleted. Here is my list of must-haves for our two little ones:
- Diapers: more than you think you will need at least to get you through the flight and the first day at your destination. In a pinch some airlines have them, and some airports, but don't count on it.
- Wipes: great for more than just diaper changes
- Clothes: a few changes of clothes for the plane; think comfy/cozy and something that will make diaper changes in those tiny bathrooms easy; some for the grown ups too if you have space, but really, who has space?
- Entertainment: toys (quiet ones that won't roll down the aisle if dropped), books, coloring/drawing supplies, ipad (toddler headphones - amazing!), etc. New items are a bonus.
- Meals and snacks: like diapers - more than you think you'll need (what if they hate the in-flight meal, what if there is no in-flight meal?); snack trap
- Milk/water bottle: (watch the pressure changes; our camelbak kid's water bottle [which, by the way, is the best we've come across in terms of leaking in non-pressurized settings] become geysers mid-flight, it's pretty embarrassing when you spray the surrounding 5 rows)
- Medicine: tylenol, diaper cream, aquaphor - whatever your go-tos are. Travel-size it if you can, otherwise I've had good luck getting things through security since they are baby items.
- Plastic bags or wetbag: lots of uses, when you need them you need them
- Blanket for domestic or budget airlines: use it as a blanket, pillow, mop up spills, play peek-a-boo
Other things you may need: formula, pump, special blankets/stuffed animals, prescriptions, and pacifiers.
With each person getting a carry-on + personal item, and lap seats getting an additional carry-on we haven't had a problem with airline allowances when also checking luggage. It does get to be challenging to carry all that around the airport plus the kids though!