Hi Michelle!
Welcome to the Disney Parks Moms Panel! How exciting that you'll be ringing in the New Year at the Most Magical Place on Earth! My parents brought me to Walt Disney World Resort for New Year's once, and while I only remember the fun we had, my mom vividly remembers the crowds. That said, Walt Disney World Resort has almost 50 years of experience through a variety of holidays and high capacity situations that will help you make your decision.
Each Walt Disney World Resort Theme Park will meticulously monitor its specific guest level on high visitor Disney Days. After certain guest counts are met, each park will limit its own future admissions though "Phased Closures." The phases are numbered 1 through 4, and while Annual Passholders, Guests staying at a Disney Resort Hotel, and those with
in-park Dining Reservations or appointments (like at Bibbidi Bobbidi Boutique) will still be let in during phases 1, 2 and 3, phase 4, which is the highest, prohibits any more admissions to that specific park for the remainder of the day. This includes those who were there earlier and left, and who have reservations for anything.
The good news is that Epcot has a very high guest capacity compared to the other Disney Theme Parks, and hasn't fully closed to all guests on New Years in over a decade. Epcot will start to crowded as other parks hit their closure phases. So definitely ask at
Guest Relations before you leave if they know (or can estimate) if both Epcot and Magic Kingdom Park are expected to hit any phases that day to make your ultimate judgement call. And if you do still decide to leave, my best advice is to head back to your Resort Hotel earlier in the day and make it back an hour or two before your reservation.
We'll See You Real Soon, Michelle!
~Kristen