Take the journey with them from start to finish. Concentrate on flying. Tell them: in the plane you first drink, then eat, then sit and then sleep. That way they know what they can expect when.
Take food and drink with you for landing and ascending. Almost all children have difficulty with the pressure difference in their ears. A swallow of water will solve the problem. For children with sensitive ears, special earplugs also want to help, but that gives a lot of crap.
Ask for extra seats when checking in and ask for the front row! Up to two years your child is not entitled to a seat of his own. But often there are chairs and young families get their own row. Just ask! Even if your children have their own seat, it is useful to take the front row (or the one below the movie screen). Then two children can play or sleep on the floor in front of the screen.
Give them different toys or something to do every hour. Provide variety. Go and stretch your legs every hour and give them something new to play with. Think of small or flat things like a coloring book.