Hello Jessica! Thanks for coming to visit the Disney Parks Moms Panel! You are asking some really great planning questions!
The general rule for the Walt Disney World Resort is that it will have very high attendance during any time that schools are closed. Many schools around the United States open following Labor Day, so this means that many families will still be visiting the Walt Disney World Resort at this time. The good news is that you are planning so far in advance that you could try to successfully plan your way around the crowds as best as possible.
One great idea that you are proposing is going to
Mickey's Not So Scary Halloween Party at
Magic Kingdom Park. The holiday parties have smaller crowds because they sell fewer tickets than for a regular park day. This would be a great way for you to experience the Magic Kingdom! Another benefit of Mickey's Not So Scary Halloween Party is that Guests holding tickets for this event are permitted to enter the park as early as 4:00PM even though the party doesn't usually start until 7:00PM.
My other suggestion for visiting during crowded seasons is to beat the crowds by arriving to the parks early in the morning and then leave the park around 11:00AM when it really starts to get crowded. Head back to your resort pool and take a nap! Then head back to the parks later in the evening when the parks tend to be less crowded again.
Jessica, you have plenty of time to keep planning! Feel free to come back with any questions that you might have as you continue thinking about a visit to The Most Magical Place on Earth!
Hakuna Matata! Jonathan G.