Hi Diana!
Mickey's Very Merry Christmas Party is tons of fun for guests of all ages! Happening select nights in November and December, MVMCP allows guests to enjoy a fantastic parade with the popular Disney character all dressed up for the holiday season. Enjoy unlimited cookies and hot chocolate at select areas in the parks as well as a special edition of Holiday Wishes!
Unfortunately, there are no special dining packages at Magic Kingdom for MVMCP's Once Upon a Christmastime parade. Although, I will say its a great idea. Hollywood Studios has a similar package for Fantasmic, but as the parade does travel throughout the entire park, you would be able to find a seat/viewing area more likely toward Frontierland, and Main Street, U.S.A, will undoubtedly be packed with guests.
As for bringing a fold up chair, Disney's policy prohibits foldable chairs to be brought into the park, including chairs similar to beach chairs.
Hope this helps you plan your magical vacation!
-Malanie