It’s fine honestly
Book a night flight if possible
Book a bassinet. If you absolutely can’t get one get a sky baby mattress you hold them in but a bassinet is much better. They work up till two I think.
Get some baby sleep spray stuff. Use it at home before you go, and then use in the bassinet so the baby associates it with sleep (also good for hotels etc)
Do as much bedtime routine as you can - into Pjs, story, bottle, and into bassinet. Take familiar blanket and toy.
Obviously the cabin will be dark at night but I think there are black out things for bassinets. Worth trying.
If you are two travelling together, book either side of the row. The middle is always the last to get booked so you might get a whole row to yourself. If someone does book, they will happily swap to sit on the ends.
More fluids than usual, the cabin is dry.
Bring plenty of familiar snack foods, don’t assume they will eat the airline food. Do order children’s meals - they are a massive point of interest if there’s no pressure to eat them.
In waking hours a tiny new thing wrapped in tissue every hour - presents are good. Familiar books and toys, plenty screen time (remember baby headphones!)
Regular supervised walks / crawls round the cabin.
Changes of clothes for baby and parents - and layers, the temperature can get v cold, but also changes a lot.
People are helpful it will be fine.