Hi Jennifer,
What a great question! Congratulations on your upcoming vacation at Walt Disney World Resort! I'm sure you will absolutely love visiting this time of year, I find the Walt Disney World Resort to be even more magical at this time of year with all of the seasonal decorations, it is absolutely beautiful! In the past I have made dining reservations on evenings where a special ticketed event was taking place in Magic Kingdom Park, however I would recommend making reservations just prior to the time
Mickey's Not-So-Scary Halloween Party begins, so that Cast Members aren't constantly asking to see your bracelets for the Party. Once you are finished dining, you will be able to exit the Theme Park and head back to your resort, or somewhere else on Disney Property (perhaps Downtown Disney?).
Personally, if I was not attending Mickey's Not-So-Scary Halloween Party, I would save visiting the Magic Kingdom for a different day. That way my family and I could fully enjoy the theme park, without feeling rushed knowing that the party will be starting and we may have to leave before we are ready to go.
Have a magical time on your Walt Disney World Resort Vacation Jennifer. Please let me know if you have any other questions while planning, I would be happy to help!
Stephanie