Hi Casey,
Thank you so much for visiting the Disney Parks Moms Panel with your question. My family has been to
Mickey's Not-So-Scary Halloween Party a couple of times over the years, and it is such a good time! This past year we made it to almost all of the trick-or-treat locations and enjoyed meeting characters with very short wait times. I also think that Mickey's Boo-to-You Halloween Parade and Happy HalloWishes fireworks are fantastic. I love the different entertainment that you cannot find at other times in the theme parks.
Mickey's Not-So-Scary Halloween Party is a separately ticketed event, so yes you do need to purchase a ticket specifically for the party. Even if you are in Magic Kingdom on the day of a party, you need to have a ticket specifically for the party or you will be asked to leave when the party begins. In actuality, I like that this event requires a separate ticket. Because the number of tickets sold for a party is typically significantly less than can be in theme park on a typical day, during the party there are lower crowds and thus smaller waits for attractions and characters. Some people chose to make the party day a day that they do not visit a theme park in the daytime hours and instead enjoy their resort, Disney Springs, or another non-theme park activity.
I hope that this information is helpful for you, Casey. Have a wonderful time planning your vacation, and please do not hesitate to ask if you have any further questions.