Hi, Lisa!
Welcome to the Disney Parks Moms Panel.
Mickey's Not-So-Scary Halloween Party is my family's absolute favorite special event at Walt Disney World! I have to say that your little guy has great taste in Disney Villains! (My favorite villain also just happens to be the evil fairy Maleficent.)
I have attended the Halloween Party multiple times; after the Villains Mix 'n' Mingle show, Maleficent typically will come down to meet Guests at the
farthest left ramp (as you are facing the Castle Forecourt Stage). You can often have the chance to meet a second Villain if you get over to their line quickly enough after meeting your favorite Villain.
I just want to make you aware that the Villains don't actually sign autographs, but they do pose for photos with their adoring fans! Villains in attendance at the Halloween Party typically include Maleficent, Queen Grimhilde (the Evil Queen from Snow White), Dr. Facilier, the Queen of Hearts, Cruella De Vil, and Captain Hook.
Wishing you magical trick-or-treating in Magic Kingdom! Please let me know if I can answer any other questions for you as you plan your trip.
Kimberly