Ignore the negativity.
Done it plenty of times. It is long. Period.
Now to practical tips since you had plenty of comments.
Look at several airlines. In my experience BA has the least empty seats on any given flight. Whereas there were empty rows in a few occasions with Singapore airlines and Thai. This might no longer be the case with the war in the Middle East. If they are empty sit, the parent wth the toddler can move after doors are closed.
Singapore as a stopover might be hot and humid but you have a beach and it is so safe. Los Angeles is another one to consider, again with the added bonus of a beach.
For the flights, the usual, screens, snacks in tiny bags, books, stickers,… I never bothered to change into pj . Seriously, there isn’t much space anyway and it is more a hassle than help.
Practice at home having to stay buckled to a parent. A chair and two belts. Parent sits on chair with a belt abound waist and back of chair , then toddler has belt attached to the parent’s belt. And make it the time for a special hug that is calm . "I love you more than apples. I love you more than a sunset. I love you more than chocolate. and then silly, I love you more than a dishwasher..,,“ . Djeco has some nice card games for very young kids. We used pouet pouet a lot . Any new game you buy, play at home a few times.
If taking a tablet, buy a headphones as well.
As others have said, a change for you and 3 changes for each kid.
Special meals are delivered before general meal service. Consider that having a kid each and a tray each is tricky, so if one of you orders a special meal, you have 1 tray, two adults, and by the time the main service starts, that adult is done eating, and you can cajole the crew into removing the tray, before the regular meal is served to the other adult. Kid’s menu is revolting. If available , fruit platter is the nicest of special meal but not every airline has it.
Have a few good plastic bags (bin bags are good). In case of vomit or poo explosion in a nappy that leaks. You have a change but where do you put soiled clothes. Have way more nappies than you think you need and an extra packet of wipes. Wipes are great to clean anything .
Last one is that luggages get lost sometimes so have with you what you might need for six hours after arrival before being able to hit the shops.
Be nice to the flight assistant and they will be nice to you.
Many airlines have snacks on a tray at the back of the plane for the night part of flight. Often they have fruit as well. And if they don’t, just ask.
Toilets can become quite disgusting. Travel with Birkenstock type of shoes for you and crocs type for kid.
Have pens with you in case you need to fill in landing or immigration cards.
And the most important tip. A flight has a start time and an end time. At some point, you arrive.