I did canada to Nz with mine at 3yo and 4mo. Thankfully DS1 love movies and is used to travelling, I dont need to pack much for him! She needs to find the one thing that keeps her occupied. Is it puzzles, favourite shows, movies, looking after a doll, threading beads on string? If she doesnt like one thing, take lots of little things to pull out randomly when things are getting fractious. Put little games etc in small ziplocs, keeps it all together. If she take a favourite toy/plushie, get a toy strap to hook to your bag/toddler wrist to prevent loss and meltdown.
Use umbrella stroller for toddler and sling for baby, backpack for the flight to keep her hands free. You can guarantee as soon as she's landing the toddler will be asleep!
Enlist stewards to help with baby if she/toddler needs bathroom. If toddler is potty trained its worth putting a pull up on them for the flight. You never know when they migh need the toilet and there is a queue or turbulence and can't go when they need it. Also helps for when they fall asleep, you don't have to worry as thats one thing taken care of.
But toddler sized headphones, crayola do a good one, for listening to movies.
Order the children's meal in advance and pack lots of snacks she knows her DC1 will eat.
I had the cot for the flight but on air nz at least the seats in that row didnt recline, it made it hard for my toddler to sleep without slipping off. Generally people are very helpful and kind and always remember the flight will end so try not to stress out even if your toddler is. If she has a layover, hang out in the club lounges even if you have to pay for the privilege. I had 10hrs in LA and paid $50 which was totally worth it. If she has a diners card Check out where their lounges are in advance.
I hope some of this is useful and that they enjoy their trip. I wish I was going again!