Hi Jennifer!
I'm so excited for you, your family is going to love Mickey's Not So Scary Halloween Party!
We have never experienced very long line-ups at the park entrance. It shouldn't take your family any more then 5 - 10 minutes to get through the bag check and the ticket turnstile. I would plan on leaving the Beach Club no later than 3:15pm. This should give you more then enough time to get to the Magic Kingdom in time for the 4:00pm admission. The best way to get to the Magic Kingdom is the Disney bus.
Some tips I like to share for the party:
Look for white balloons throughout the park to find the trick-or-treat locations.
Trick-or-treat lines get shorter as the night goes on.
The second parade tends to be less crowded then the first.
The Merchant of Venus store in Tomorrowland has the best selection of event-themed merchandise.
Photo pass photographers hand out special event photo cards that night. Any photos you take that evening can be purchased for 40% off.
Thanks for your question and have a magical vacation!