Olivia,
I will confess that I don't keep that close of an eye on how often the Halloween Party sells out, but I have heard that the ones at the end of the run do sell out, especially around the actual date of Halloween itself. Since that is the last one, I would assume that it will sell out, so I would STRONGLY urge you to buy as soon as you are certain of your number of guests.
Here is where I would go, assuming you did choose to go to that party;
- October 31 - Magic Kingdom
- November 1 - Magic Kingdom
- November 2 - Hollywood Studios
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November 3 - Epcot
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November 4 - Hollywood Studios
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November 5 - Animal Kingdom
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November 6 - Magic Kingdom
I think this is a good mix, though you might keep your last day (the 6th) open for a change of parks. With children both 7 and 16, they might say that two days at the Magic Kingdom were enough, and opt for another day at Epcot. Besides, at that time of the year, a little spontaneity won't kill your fun, and might take a bit off the stress off.
I hope that this helps! Write me back with more questions!
Have a magical vacation!