Hello Michelle! If I'm not misunderstanding, you are saying that you have tickets to the Magic Kingdom during the day and also have Mickey's Not So Scary Halloween Party tickets for later that evening? In this case, yes the Magic Kingdom will close at 7 p.m. to regular day guests, however those with Mickey's Not So Scary Halloween Party tickets will be able to stay and enjoy the park without having to leave and return at a later time. You may need to go to the front of the park to receive your Halloween Party wristbands, but there will be no need to actually leave the property. During the hour from 6 p.m. to 7 p.m. there will be lots of day guests trying to leave the park and lots of party-goers arriving to the park, so I suggest scheduling dinner for around that time or spending time on the attractions to stay away from that crowd. Cast Members will be checking for wristbands throughout the evening.
I am planning on attending the Halloween Party as well, and am especially excited to see the
Hocus Pocus Villain Spelltacular. Hocus Pocus is one of my favorite movies, and I'm so excited that the Sanderson Sisters are back causing mischief. Also, don't miss Mickey's "Boo-To-You" Halloween Parade. It is my Mom's Panel friend Jason K.'s favorite thing to do at this special event.
If I misinterpreted your question, please feel free to ask again providing specific information. I hope you enjoy Mickey's Not So Scary Halloween Party, and beware of hitchhiking ghosts! Have a magical first Walt Disney World vacation!