Boo to you, Beatrice!
I am excited because this year will be my first time attending
Mickey's Not So Scary Halloween Party myself, and I can't wait!
Your question enters us into one of those "Twilight Zones" of Disney Park questions and answers. I have been doing some research on this, though, and will share what I've found out with you.
The official beginning time of Mickey's Not So Scary Halloween Party is 7:00 p.m. Technically, Disney does not have to allow anyone to enter the Park until that time. However, they have been known to do so as early as 4:00 p.m. in the past.
This being said, I, personally, would not make my dinner reservations based upon anything but the official times, 7:00 p.m. - Midnight. That way you will not be disappointed if Disney is sticking to the official times on the particular day you choose to go.
So, if you do not plan to visit the Magic Kingdom, using your Park Pass, before 7:00 p.m., you could make an Advanced Dining Reservation for one of the wonderful restaurants located outside the Parks, such as Chef Mickey's at the Contemporary Resort. Or, if you are late night diners, make your reservation within the Magic Kingdom for a little bit after 7:00 p.m. That way, you aren't taking any chances!
Enjoy the party, and make sure you save some room for candy!