Hi, Angel!
Thanks for reaching out to the Disney Parks Moms Panel! It looks like you have a fun visit planned at Walt Disney World Resort!
I think the pro reasons to visit Magic Kingdom Park the same day as
Mickey's Not So Scary Halloween Party is you won't have to rush over to the park after visiting somewhere else. Since you will have already spent the day in the park, you will have an idea of which attractions you'd like to go on again. Plus, you will have more time to enjoy seeing characters and trick or treating and not feel the pressure of hitting as many attractions at the same time. The other positive is you will not have to leave one of the other parks early - Disney's Animal Kingdom Theme Park and Epcot close at night. It is likely you would have to leave one of those parks early so you can enjoy the full Halloween party experience.
The con of visiting Magic Kingdom Park all day before attending Mickey's Not So Scary Halloween Party is you are paying twice to visit the same park. In the back of my head, that would kind of annoy me. But the party is so much fun, and it is neat to see how the "mood" of the park is different. There is a park transition when guests who did not purchase the party ticket leave for the day.
I hope those thoughts help as you plan our your unforgettable visit to Disney World! Let me know if I can help answer any additional questions you have before you arrive!
-Jen