Hi Yolanda and welcome to the Disney Parks Moms Panel.
Yes, all Guests without Special Event Tickets to
Mickey's Not-So-Scary Halloween Party will be required to leave the
Magic Kingdom Park at 7:00 PM on days when Mickey's Not-So-Scary Halloween Party will be taking place.
If you are only planning to spend 1 day at the Magic Kingdom Park during your trip, I would recommend visiting on a day when Mickey's Not-So-Scary Halloween Party is not taking place if at all possible, in order to maximise the amount of time that you will be able to spend at the Theme Park.
Alternatively, if you have never experienced Mickey's Not-So-Scary Halloween Party before, perhaps you could consider purchasing Special Event Tickets so that you can stay at the Magic Kingdom Park on the day of your intended visit and soak in all of the Halloween fun on offer during the party.
I have to tell you that Mickey's Not-So-Scary Halloween Party is my family's favourite Special Event at the Walt Disney World and we definitely wouldn't miss it should it be taking place during a visit.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma