Hi Lindsey,
Thanks so much for stopping by the Disney Parks Moms Panel! Great question!
Mickey's Not-So-Scary Halloween Party is my favorite special event of the year at Walt Disney World. It's definitely worth experiencing if you've never been. Special events like this do require that you purchase a separate ticket for admission. In past years, Guests with tickets to Mickey's Not-So-Scary Halloween Party have been able to enter Magic Kingdom as early as 4:00 p.m., even though it doesn't officially begin until 7:00 p.m. If you plan to be at Magic Kingdom earlier in the day, before 4:00 p.m., you will also need to purchase a park ticket for the day.
One option to consider is to just get tickets to Mickey's Not-So-Scary Halloween Party and arrive right at 4:00 p.m. Since the Party runs until midnight, that will give you 8 full hours of frightful fun in Magic Kingdom (and save you the cost of 2 tickets per person for a single day).
Please let us know if we can help answer any other questions as you make your plans, Lindsey.
Have a spook-tacular time!
Beth