How long is the flight?
Because of having family in different parts of the world, I've been doing 12h flights with my son since we has 2.5 years old.. (not including the additional shorter connection flights.
Tips:
Pack some "odds and ends" in a backpack pocket.. little things that they can fiddle with in airport queues. My son loved and still asks for this. They can be toys you already have, or you can go to a cheap toy shop and just get a few things for them to fiddle with. It makes every queue and wait 10x better if they have something small and exciting to be fiddling with while waiting.
Plan your carrying solution. Personally I never liked prams, so I've always done a mix of carrying (tiring) and making sure I have a suitcase that they can sit on while we are waiting. Look out for airport trolleys that have kids places to sit. Some airports have great ones with little ridable cars in front that kids can sit in.
Find the play area in the departure lounge.. more and more airports have soft-play these days, although it might be a hike from your gate.
For the flight, bring time-consuming activities. Finger foods that take time to eat (my son loved breadsticks with a dip.. and I loved it because he would take ages to dip.. bite.. dip.. bite..dip.. bite.. etc.) Water colouring pens (the ones that you just fill with a bit of water and then brush over the page to reveal the colour), and magnetic pads with magnetic tiles etc. were all big hits. I'd fill a backpack with all sorts of odds and ends, and then after that its just a case of keeping the time filled.
Activity...
Airplane walk...
Snack...
Activity...
Reading...
Toilet...
Activity...
etc.
For longer flights you can add in screen time + sleeping + meal times.
It really isn't all that bad.