exhaust them at the airport before ypu get on the plane.
dont feed them meals, snack snack and snack for as much of the flight as possible. Snack that are eaten in small bits like raisins
get one of the inflatable footredpst thingies that fills the footwell so you can make a bed up for him…I am assuming you have bought a seat for toddler (if you havent its going to be hell)
pack nappies and wipes in one use bags,so you just grab one to go and change him and dont have to take the whole carry on bag into the loo
extra clothes for you and toddler, ther eis a high chance of vomit, poop explosion or spilled drink
if you have a sippy cup, adjust the pressure in it after take off or it will explode everywhere..also be cautious with pouches of food etc
lots of small toys/stickers/new things and only take them out one by one
ipad, headphones (you can get ones on a soft headband that are toddler friendly) and back up batterypack. If you can charge it, do so
get a carryon bag that opens up widely so its easier to find everything, and individually bag stuff inside it to stay organized
take your own sealable bags to use for rubbish/poopy clothes etc
pray