I used to fly to Portugal regularly and my 2-year old boy was a handful on those short flights. Then we had to fly all the way to New Zealand, and I was dreading it. A friend who'd taken her twin toddlers to Australia gave me some advice, and it turned out to be golden.
She said: take a big bag filled with toys and snacks and books that they've never seen before. Don't neglect the snacks, lots and lots of interesting little snacks.
What I found was, mini-cheddars were good, little packets of assorted nuts & raisins were fantastic, and those long sticks dipped in chocolate called "Pocky" were AMAZING. (Don't go for all sugary things though ofc)
Also, you don't need an iPad, a generic tablet will do (we got one on Amazon for £99), and the free demo games "Kids Doodle" (a painting app), "Lego Trains", "Little Things" and the CeeBeeBees app were all very well received (he still plays them, months later).
We scheduled our three long (7-11 hours) flights to be in the daytime, stopping in airport hotels each night. Definitely worth it! We flew Emirates (purely for their handy schedule) and the cabin crew were very kind to us.
We also warned our boy well in advance what to expect, we're going in a plane, up in the sky, for a long time, all day, have to be quiet, other passengers might be sleeping, etc etc. Also warned him what landing would feel like, boomboomboom, then brake brake! That was worth it because he enjoyed rough landings when they happened!
Last tip, when landing, always make sure they are sipping water as you lose altitude, otherwise they get sore ears.
All in all the experience was a lot better than I expected :)