Hi Shannon!
Thanks for your question and for visiting the Disney Parks Moms Panel!
The short answer is, it depends.
Mickey's Not-So-Scary Halloween Party is a separate, "hard ticket" event at
Magic Kingdom which means that it requires a separate admission charge from regular attendance at Magic Kingdom during regular operating hours.
To hopefully clarify, if you were at Magic Kingdom for the day on a day when there is also a MNSSHP happening, then you would also need a Party ticket to stay beyond the official 7 PM Party start time.
If you just wanted to attend the Party and NOT use a park ticket day, then you can enter the Magic Kingdom with your Party ticket as early as 4 PM and stay until the Party ends at midnight.
Attending Magic Kingdom for the day WILL NOT automatically include Mickey's Not-So-Scary Halloween Party.
I hope this helps and I wish you a Magical trip!!
Chris