Ooh have a lovely time wherever you’re going.
on long haul planes there are a lot more toilets and as it’s a lot longer flight I found there were never/rarely any queues except just after take off/just before landing, so just be prepared. Change nappy just before you board (and don’t bother boarding first, you will have plenty of space for your stuff, it’s not the mad rush you have like flying with Jet2 to the med etc). Then you should be golden for a while.
take multiple changes of clothes for both of you!! And layers some flights are cold some are warm!
load up phones/iPads with stuff to watch, and or get them used to headphones so they can watch things without winding everyone up.
some of the best toys I took that kept mine entertained for ages were the results me sticker books (packs of 3 on Amazon), SULOLI 8pcs Pop Tubes Sensory Fidget Toys For Children or Adults, Autism and Special Needs. Reduce Anxiety and Stress https://amzn.eu/d/5PW55CL these were the favourite by far, both my 3 year old and 1 year old still play with them in the car now! I took a small Tupperware of playdoh in sandwich bags rather than the normal tubs with a few cutters.
snack wise, I’ve done both the snackle box (like a Tupperware with lots of compartments) and had each filled with raisins, yoghurt covered raisins, mini breadsticks, mini cheddars, chocolate stars, orange segments, Pom bears etc, and I’ve also just taken packets of all of them and let my little ones have whatever whenever they fancied. As long as they’re chewing and swallowing (even a drink) on take over and landing they’ll be fine.
I saw someone on a flight to Singapore pack all their kids snacks and toys in one of those fold out/hang up wash bags you can get like this Pystuvo Hanging Toiletry Bag 9 Pieces,Travel Hanging Toiletry Wash Bag with Travel Bottle for Makeup and Washing Accessories, Men and Women for Travel or Daily Use,Large Travel Wash Bag,Pink Panther https://amzn.eu/d/2GieyPD then hang it from the tray table hook thing so they could access what they wanted easily which I thought was genius.
advice wise I would say, just accept it’s going to be long and not the most fun you’ve ever had, accept there’s going to be lots of screen time and random snacks at random hours. You’ll get back on whatever schedule/eating standards you normally have when you’re away/home. Do what you got to do to get through the trip.