You'll be fine. I promise!
It does go much quicker than you imagine. By the time you have taken off, had a snack, read a few books, walked to the toilet and back it's almost time to land again. It will all be worth it at the other end. I kept DD2 awake yesterday and she crashed on takeoff, it was perfect. Maybe stretching her sleep out will work in your favour. There is the possibility it could all go horribly wrong, but i find when i'm more relaxed about the whole thing the kids travel better.