Hi Charlene,
I love your plan! I think you're in for a fantastic dinner at
Be Our Guest Restaurant and ending the evening with
Mickey's Very Merry Christmas Party sounds positively enchanting!
Guests have been allowed into the park as early as 4 PM with a party ticket in past years, so assuming the schedule doesn't change for this year, you'll have plenty of time to get to your dining reservation. I recommend staying up to date with the
Disney Parks Blog posts as the parties begin to make sure your timing works for this year's celebration.
Once that is settled, I would aim to arrive at Magic Kingdom Park at around 3:45 PM, to allow time for going through security checks and obtaining your party wristband. That way, when Cast Members start allowing party Guests to enter the park, you'll just walk down Main Street, U.S.A. and right to Beast's castle in plenty of time for your reservation.
I hope you have a wonderful holiday at
Walt Disney World, Charlene. Hope to see you real soon!
Andrea U.