Jillian,
First off, you are going at a great time, so you should take advantage, not avoid, Extra Magic Hours. As for the Halloween Party, it is probably more targeted to kids younger than yours, but still fun if they are into that holiday. It will also gain you shorter lines to major attractions, but at an obvious price. In your case, with a longer than typical vacation, I would say you could avoid the cost.
So here is where I would go, with a few water park stops in there;
- September 20 - Magic Kingdom
- September 21 - Epcot
- September 22 - Water Park (am) / Animal Kingdom (pm)
- September 23 - Magic Kingdom
- September 24 - Water Park (am) / Epcot (pm)
- September 25 - Hollywood Studios
- September 26 - Magic Kingdom
- September 27 - Hollywood Studios
This gets you in the Magic Kingdom 3 times, Epcot and Hollywood Studios twice each, the Animal Kingdom once, and two water park visits. The tough part of the water parks is that they don't open until 10 am, but I would plan later nights out the nights before, sleep in, have a good breakfast, then swim to later afternoon. Then you can have some great evenings in parks with evening Extra Magic Hours!
I hope that helps?
Have a magical vacation!