It's going to be rough, but you know that, and you're committed now. Yes, full change of clothes for everyone, extras for baby and toddler. Water bottles you can manage one handed for everyone except the baby. All the snacks. I would include quite a few snacks for the adults because actually eating your meals may be difficult to impossible depending on what the children are doing at the time. Swap children between parents when you can – 'a change is as good as a rest', well, it isn't, but it's better than nothing. I carry on one small sized suitcase for extra clothes, snacks, and toys, plus a rucksack each.
Watch something/read books about going on a plane with the toddler, or just pull up some images online and talk through what will happen. Airports are quite variable on how they handle security with small children – some are brilliant, and some frankly have no clue (cheers to the security attendant who wanted my 4 year old to be last through, rather than going in between me and her older sister – leaving her standing on her own in an unfamiliar, busy place was a great idea).
Exercise the toddler in the airport. Check if there are play areas in the airport (Changi has multiple, but Changi is also huge), but even if there aren't, let them walk, or if you find a quiet area, run about a bit. A good view of airplanes landing and taking off is also good, lots to see and talk about.
Take books where you can talk about what's in the pictures or ask toddler to find the X as well as just read the story – you get a lot more time out of each book that way. Sticker books and magic colouring (either water or the pen sort) can be good, a few little animals and/or vehicles if they'll do that sort of pretend play. A set of stacking cups can be a good compact toy, building, stand-in tea set, use in the bath or at the beach when you get there. A pack of those little post-it notes you used for marking pages in books – tiny to pack, but toddler can stick them all over (seat, window, tray, you...), and they come off easily. Can make chains of them for bracelets/necklaces/crowns if they like. A travel set of magnetic tiles if they like building – one in a metal box, so the box becomes the building surface.