Hi Melissa,
Welcome to the Disney Parks Moms Panel. I am a huge fan of
Mickey's Not-So-Scary Halloween Party and I hope you love it as much as I do!
If you decide to spend the day in Magic Kingdom Park before attending the party on the same night, you will not need to leave before the party.
You'll notice Cast Members around the park setting up stations sometime in the late afternoon to hand out party wristbands. Just stop by one of these stations to have your tickets scanned and you'll be able to get your wristband and party map for an evening of spooktacular fun!
When my family did this, we got our wristbands in front of
Monsters, Inc. Laugh Floor in Tomorrowland, but locations may vary depending on your party date.
It sounds like you are planning a wonderful vacation, Melissa. Please let us know if you have any other questions.
Andrea U.