For babies feed milk (breast or bottle) on takeoff or landing. It helps with the pressure in their ears. For older children bag of chewy sweets.
Go to the pound shop and buy half the toy aisle. Sticker books, small plastic toys, etc. Whenever dc start getting antsy just produce a new toy or book.
Dowload their favourite TV shows on ipad and use over the head earphones for preschooler. Also get a few games on there.
Try and book nighttime flights. That way they'll sleep for most of it hopefully
Raisins make excellent snacks. Small and bitty so can take a good 15 minutes to eat one by one.
More nappies than you think you'll need (sometime the cabin pressure can somehow lead to the most explosive poos). Full pack of wet wipes.
Change of clothes for you, 2 or 3 sleepsuits for the baby, one for the preschooler.
A baby sling for the airport. I used to pile our hand luggage on the pram and carry ds. Whist dd stood on the buggy board. It means that if you have to walk to get the pram from baggage reclaim at the end of the flight you still have your hands free.
I used to take 2 bags for hand luggage.
One bag was packed with kid stuff, toys, games, a few muslin squares, nappies, wipes, snacks.
One was packed with the changes of clothes, the sling and a couple of pashminas (the make excellent blankets).