I would not be taking other people's children for a start, see if their families might want to go at the same time if you really think the kids will need the company. 2 weeks of taking responsibility for other people's children. Tickets at a minimum cost of £437 per person, the cost of additional flights, additional food, additional health insurance is probably £2000 per person, if you're doing it on the cheap.
Don't book it until restrictions are completely lifted if you're only doing it once. The fireworks are spectacular but they're currently not doing them because of Covid and crowd control. Many restaurants are not open, fast pass is suspended and disney dining plan is currently not available.
Book things as early as you can to get the best availability.
Book the attractions in Pandora (in magic kingdom) around sunset. As the sun goes down, the whole place lights up. To see it around you is absolutely amazing.
Don't underestimate how big it is, and how tired the kids will get. When we go, we usually like to get to the park were going for the day, stay until 12/1. Then we often head to the water parks for the warmest hours of the day. Often get food outside of Disney, then head back to another park in the evening. We plan to do the fireworks or closing show at each park once, the other nights, by about half 7 you generally find that the wait times of rides go down quite a bit. You can almost walk straight on some rides as the fireworks are on.
Mickeys kitchen sink is a must for a group as big as yours!! Its at the beach club!
If any of you are disabled, make sure you take their blue badge, from my understanding they now charge for parking in their resort as well as at the parks, you should, from my understanding be able to bypass the resort parking fee with it. Also, see about a DAS pass (you mentioned a change in DD9s health- I took that to mean worse so that's why I've said that!)
If your DD is into Princesses, take her a Disney princess outfit or two! They can wear dress up in the parks until they're 12!
If you're staying offsite, and you choose a villa- book one where you'll know the area before you arrive. Last time we went we ended up in a villa out in Haines City. Between the location being so far away, and the traffic on the I4 it took us more than an hour most days to get to Disney.
You can get free water in the parks, but honestly, I do a walmart trip when I can't sleep early hours of the first morning. I always buy a few crates of drinks, I usually freeze them and take them out of the freezer last thing at night the night before.
Don't overlook the shows. I hated lion King as a kid. Hated it, haven't much liked it as an adult. The lion King show, makes me cry every time I'm there. Dole whip is available just opposite =)
Don't miss the safari in animal Kingdom. If you can do it at a few different times. The experience just differs a lot.
Sanaa at animal kingdom lodge is really quite special.
Ohanas for best friends breakfast is something we love!
Beauty and the beast fans will love Be our guest. It's fairly pricey, but its QS so no tip is required. It's such an experience being at the restaurant. The masters cupcake is also available on the menu at around $5. You see the beast if you go for dinner, but its significantly more expensive.
Don't forget the sunscreen. Usually we can tell fellow bits by their skin tone. Many of us end up lobster coloured!! If the kids do happen to get burnt, there's an after sun in an aerosol bottle, it contains Lidocaine? It numbs the area and is fab! I got burnt because I applied sunscreen like I was in the UK. Hahaha. I BLISTERED!
The teens will really enjoy disney springs. Everglazed, amorettes, Gideons bakehouse, or sprinkles are all great for a sweet treat. However gideons bakehouse allows upto 6 cookies per customer, so two of you might need to queue. The kids will love the Cupcake ATM
If looking for a really cheap, really simple, fries and burger meal, I'd recommend steak and shake, it's literally cheaper than mcdonalds. Their milkshakes are amazing! They also do a decent and inexpensive breakfast. They're offsite, but if you end up staying offsite it's not a bad shout for a quick, inexpensive meal.
Also offsite, but Longhorns for steak.