My DD has been through long flights when young (we flew transatlantic and then a short hop when she was 5, and many more, including 2 more transatlantics, since then).
One thing that helped was having a few parcels in my carryon. She could rummage and pick one every hour to open. They were wrapped up, and had things like crayons and colouring sheets, playdoh-type stuff, a small toy set, a kaleidescope, a magic drawing thing (plastic that you "write" on with a plastic stylus to draw lines/pictures and pull out to erase), a pack of cards, new edition of favourite magazine and a new travel game. Tiger is great for travel bits and bobs or Poundtretcher type places.
Magnetic travel games are great - ludo, snakes and ladders, monopoly, connect 4, hungry hippos etc.
Bring snacks and let them help choose them.
Let them buy a book/magazine in the airport to read on the plane. And while you buy water, let them buy a treat (drink or sweets/crisps) for on board as well. Relax over hot chocolate if you have time, and try to make sure you have time to keep stress levels down (both in stopping to enjoy and also not chasing through security and running for planes).
Another thing I found later on (and also after DD's ASD DX) was that Manchester Airport has an autism guide. Before you scoff, it is a very clear guide to children about what to expect going through an airport and getting on a plane - including lots of pictures of what to expect to see and talks about noisy places and lots of people. Dublin Airport also has one, and I suspect that more airports have them now as they are so uiseful. So have a look at your planned airport's website for one, and if not, look at another airport and say it will be like this but may look a little different in our airport. But it takes the fear out of lots of the process - my DSis has used it for her (non-autism spectrum) DCs and loved it too.
Plan your bags too - in your carryon, have a change of clothes for everyone (accidents can happen, airlines can lose luggage etc). But try to have 1 bag that goes straight up overhead (spare clothes etc), and another that goes at your feet with everything you need on board - snacks, distractions, your own book etc. Headphones are also great, and tech like iPads or DVD players. And let each DT have a small bag with their "treasures" - maybe own favourite bear, an extra jumper in case it's cold on board, a small snack to use themselves (maybe even a small pack of jellies?).
Enjoy.