Make sure that you carry any passports (including visas), medicine and/or vaccine information, and back up birth certificates/social security cards directly on you. Make sure to consult with a travel clinic doctor for what vaccinations they would recommend for your child. You probably don’t want to let your child at 10 years or below carry anything since all of the above can be very difficult to replace in a quick manner. It also might be a good idea to have a first aid kit with you for those times when you’re traveling to the airport or in between flights/stops.
One good suggestion might be to bring a chewable candy (like milk duds or starbursts) for popping their ears when the plane takes off and lands. This might make the plane ride more fun for nervous little passengers and will keep your child from swallowing their gum. There are also plastic earplug types called Earplanes that help keep the ears open with the change in air pressure. They have children sizes, starting from toddler, so it might be good to bring these along in case your child has trouble popping their ears.
Another important thing might be worthwhile is ensuring that all your children are in layers. The temperatures will change drastically inside the plane from inside the airport so having an easy way to get warmer or cooler could save you a lot headaches. It might also be a good idea to put your child in sneakers with socks instead of crocs or sandals as that might help with temperature regulation and prevent things like tiny toes from getting stubbed.
***TIP: Sometimes airports will provide special children meals for children flying on their airline. Check ahead on the website before flying.***