Don't worry too much about it! The flight attendants are usually very attentive to children and it helps to have them on your side.
We did a lot of travel with our twins before we moved here (they were 4 when we came) and I found that kids are excited at first by the plane, but then they tend to sleep.
My favorite distraction toy was one I made out of pieces of colored felt- I just cut out lots of shapes and put them in a zip lock baggie. Then I put a piece of black felt (with a sticky backing- from the craft store) on the back of an old picture book. Felt can't be thrown far and won't roll away under seats. Lots of people suggest having new toys- I agree. But an old comforting toy is also needed.
AirNZ has little kid friendly inflight entertainment- Wiggles, etc.
This site has some good tips:
http://www.twinslist.org/flyAir.htm
Saline nasal spray can help with the dry air that tends to make little noses stuffy. Drugs weren't something we ever tried.
Do you have a backpack carrier? I swore by them for hands-free passage through the airport and right onto the plane. But by 20 months you might be too big for that. I guess you'll be getting them tired by running!
In general kids are more comfortable on the flight because they can wiggle around in their seat and get into different positions that we can't because we are too big. You'll be excited about this move, so kids naturally pick up on that excitement too. Take photos and celebrate your success!!