If the party starts at 7pm they usually close the park at 5pm to general public. If you are already in the park at that time they will allow you to stay and they will send you into one specific area (when we want it was liberty square that they kept open). This is great because you get an extra hour, no queues on any of thr rides in that area and you can usually get your complimentary family photo done early.

We didn't spend the day in Magic Kingdom but turned up around 4.30 with our annual passes. However, because we were in fancy dress and we had the ticket for the event they let us in at that time so it is worth trying.

I am sure you will all have a great evening - it's fantastic!