Hi there Tamara, and welcome to planDisney!
Firstly, as someone also sharing a birthday with your daughter next week I'd like to wish her the Happiest Birthday ever - there's really nothing like turning older at the Most Magical Place on Earth and I know I would've absolutely gone wild if I got to celebrate my birthday at Walt Disney World Resort - complimentary birthday button and all!
It sounds to me like this
Halloween thing that you mention must be
Mickey's Not-So-Scary Halloween Party taking place on the evening of your daughter's birthday; I can definitely see how this does inconvenience families who may have already had their plans set before the
Hours & Events Calendar updated to show the reduced hours with Magic Kingdom Park closing to day guests at 6:00 P.M. as the party continues on exclusively for attendees until midnight. (In my opinion, one of the biggest drawbacks to this is that folks and families who may have been unaware won't be able to watch
Happily Ever After fireworks that evening due to the event.)
Since this is a
Special Ticketed Event, guests have had to purchase admission separately in advance in order to receive special wristbands that allow them to remain; I've observed that Cast Members slowly clear out sections of the park to enforce who is eligible to be in Magic Kingdom Park that evening and gently usher day guests on their way. At the current time it's
completely sold out for August 27, but you're always welcome to speak with the helpful folks at
Guest Relations to ask what options they might suggest for you; this could mean looking at the possibility of choosing a different theme park for that day or by utilizing
the Park Hopper option to finish off the evening at EPCOT instead.
I've personally encountered this situation myself, and what ended up being a shiny silver lining was that the wait times in standby queues across the rides and attractions were much lower - simply due to so many other families choosing to avoid Magic Kingdom Park and its reduced hours that day. (This has the potential of helping you get so much more done in less time!) What's also great about celebrating any major occasion during a vacation to any Disney Destination, is that you can keep on celebrating your birthday all week long if you wanted to - so if it doesn't necessarily work out on the 27th itself I know that as a kid I would've loved the idea of having a "Bonus Birthday" the day after!
I hope that these suggestions help you plan ahead for next week, Tamara! Here's hoping the birthday girl has the most awesome time turning eight amidst the spookiest season here at Walt Disney Disney World Resort!
May all of her future birthdays be just as magical!
Prince