I recently did an 8-hour trip to the Caribbean with my 2.5DS and, despite sleepless nights dreading it (I posted on here for tips too!), it wasn't nearly as bad as I thought it would be! So try not to be toooo fearful! I thought my DS would be running up and down the aisles the entire time, but actually he enjoyed being in his seat (a first!).
I downloaded tons of CBeebies onto the iPad, as well as apps, and that really helped. He also loved watching the children's in-flight entertainment.
I went to Fara (the kids' charity shop) and bought about five 'new' toys which I gave him at strategic intervals during the flight and which kept his interest the whole flight.
I kept snacks - raisins, hula hoops (not too much sugar) - and water close at hand. And for his ears I gave him a lollipop for take-off (although he ended up sucking his dummy which was good). And he loved getting his aeroplane food (make sure you book the kids meals for them - they don't do it automatically).
I also bought him a Pillow Friendz pillow (a tiger) which was brilliant the whole holiday. Nice soft place to rest his head on the flight as the aeroplane pillows aren't comfy, and you can attach them to the armrest.
And the BIGGEST advice I can give you is that you need to try not to feel stressed. (Easier said than done I know). The plane's engines are actually quite loud - so even if they're kicking off big time, even a few rows back you can barely hear it. Seriously. AND most people are sweet to kids and smile and interact with them. You do get the odd meanie who looks grumpy or sighs, but really, who cares. You'll never see them again. And you can always smile kindly and say I'm so sorry, but I'm trying my very best. If you have any ideas, then do please share them.
And remember - IT WILL END. And Children have just as much right to fly as anyone else.
Good luck!!!