Hi! I did this same trip when my DC had just turned 2. It was honestly not terrible at all, I was quite nervous. It was just the 2 of us, I assume from your 'we' there is your partner/spouse travelling too?
My main tip was, don't expect to relax. I really had to change my thinking from 'this will be hell' to 'there's no other option' and lots of 'how lovely to have a chunk of time together' (I was working a lot at the time and had lots of mum guilt, so tried to turn that around into lots of quality time with my child).
If there's 2 of you, I really suggest you take turns. 2 hour blocks. OR a movie each. So one adult can completely zone out and relax. Let them have a movie, nap or read for 2 hours by the window seat. Don't put DC in the middle, or if you do, be firm with 'leave daddy/mummy/whoever, they're relaxing' and really do let the other adult relax. You're likely to then both feel happier when it is your 'turn' to be 'on'.
I worked hard to ensure my DC would sleep when appropriate, and accepted that this may well be on me. I didn't expect to sleep - I would rather be tired with a ratty toddler!
I took lots of tiny presents/activities and wrapped them all in newspaper. Someone suggested that as a) it adds 20 seconds to each item coming out and b) you can use them more as bribes. Buy whatever your child will like. Arty things, small figurines, a book, whatever. We did lots of sticker books. I bought one of those magnet sets - ours had ballerinas. We walked up and down a lot. Went to the little station at the back for drinks/snacks on the longer flights. Read stories. Watched tv. Talked a lot!
I took empty bottles and asked for regular milk to help my little one get to sleep at bedtime and naptime. Took a sleepsuit and did a 'let's get ready for bed' routine for 'night' with a bedtime nappy and went to brush our teeth in the toilets and a sleepsuit.