Make sure they pack a carry-on that they can actually maneuver through the airport. We prefer having her use a small bag on wheels. Said bag only gets heavier as they add in keepsakes.
Wear long stretch pants/sweatpants. Long flights get chilly and she liked being in pants with easy to remove shoes (i.e. Crocs or slip-ons).
Plane food is notoriously unappetizing. Pack snacks that they’ll like such as goldfish crackers, granola bars, bagels and fruit. I try and stay away from chocolate and sweets on a plane.
Pack hand wipes in your carry-on to wipe down the seat's tray table and seat arms. It never fails that she dumps her snack out directly on the tray table.
If you're going to a destination with a pool, pack a swimsuit for each of you in your carry-on. That way if your luggage is lost you can still enjoy yourself upon arrival.
Pack in Ziplocs: This has been a game changer for us. I pack all of her bathing suits, underwear, socks, shoes, goggles and hair accessories in separate plastic storage bags. Diapers and swim diapers could also be in a separate bag. That leaves me with just a pile of shirts, shorts and a couple of dresses for her. Now it's super simple to unpack and find specific items. I also pack an empty garbage bag that we use for dirty laundry along our travels.
Travel with medicine that you know works for them.
Set clear screen time: Make it clear before you leave what the screen time rules are going to be.
Every morning lay out the plans for the day. It helps them feel secure and confident when they know what's ahead.
Build in rest/pool time. Let's face it, most young kids don't love a full day of sightseeing! We also allow some screen time before dinner if it's not coming out with us.