Hi Aimee!
Welcome to the Disney Parks Moms Panel! How exciting that you'll be visiting Walt Disney World Resort during the
Most Wonderful Time of the Year! Don't forget to check out the festive Holiday Kitchens that are all around Epcot as part of the Epcot International Festival of the Holidays!
The festivities of
Mickey's Very Merry Christmas Party, like Mickey and his friends donning their special holiday outfits at their meet and greets as well as holiday stations handing out cookies and beverages, kick off right at 7:00 P.M. Over the last few years, party guests have been able to enter Magic Kingdom Park with only a party ticket as early as 4:00 P.M. Disney is notorious for change, and this could change in the future. So definitely check with the concierge at your Disney Resort Hotel, or with a Cast Member at any Guest Relations location, if guests are indeed able to enter at 4:00 P.M. this year.
If you can enter at 4:00 P.M., the answer about getting in a bit earlier to make your
Dining Reservation is that your milage may vary. Walt Disney World Resort is the Most Magical Place on Earth, so I would think that entering a few minutes before to get to your reservation wouldn't be an issue. However, it does depend on the Cast Member at the gate. To be super sure, you could try to change your Dining Reservation to a later time during the party, or to a location where theme park admission isn't an issue, like at one of the nearby Resort Hotels. However, if you are able to enter at 4:00 P.M., you should be able to make this meal as each reservation does have a 15 minute late arrival grace period.
We'll See You Real Soon, Aimee!
~Kristen