Greetings Clayton-Owen!
I think booking an appointment at the
Bibbidi Bobbidi Boutique prior to Mickey's Halloween Party is a phenomenal idea!
That way, your royal princess can get all dolled up in her favorite princess dress and look picture-perfect before the
Halloween festivities officially begin at 7pm!
As long as there's an appointment window available during the three hours you're allowed into the Park before
Mickey's Halloween Party begins, you should have no problem. The number to the reservations line is: (714) 781-STYLE or (714) 781-7895.
I would personally try to book an appointment any time after 4:30pm on the day of your visit.
I say this because you'll need some time to get through the turnstiles and into Fantasyland (where the Bibbidi Bobbidi Boutique is located). If you book something earlier, your Mickey's Halloween Party tickets will not allow you to enter the Park prior to 4:00pm so you may risk losing your appointment.
I hope this helps!
Thanks for visiting the Disney Parks Moms Panel.
Have a Ghoul' ol' Time!
-Anabel M.