I flew recently with my 20 month old from US to Spain. Lots of new toys helped. My son has a tendency to play with a new thing for 30-45 minutes, and then never again. We actually had to make a trip to Toys R Us in Barcelona to buy more toys for the ride home (coloring books, legos, match box cars, etc...). This ate up a lot of chunks of time.
Accept the fact that your youngest may cry or be uncomfortable. Do your best to calm and soothe them, but it is bound to happen.
Sleeping in a new place was an adventure. My son is not a great sleeper at home, so when we put his in his fancy lie-flat seat, he was hardly as excited as I was on my first international business class trip. Just have a backup plan for sleep -- obviously this is kid-dependent. We ended up having my wife shush in his ear while I rubbed his back and he eventually gave in and went down.
