Hi Elizabeth!
You are going to Disney at one of my favorite times of the year!
It has not yet been announced on when the
Mickey's Very Merry Christmas Party (MVMCP) tickets will go on sale. However, historically, the sale dates have been early May. If you haven't already, you may want to visit sites such as Allears.net, Intercot.com, Passporter.com and sign up for their free newsletters. As soon as they know when the tickets will go on sale, it will be announced on their websites and in their newsletters.
Once the tickets go on sale, you can secure yours by calling 407-W-DISNEY.
As far as a character meal, are you looking for one inside the Magic Kingdom? If so, the only two available are Crystal Palace (Pooh, Tigger, Eeyore, Tigger) or Cinderella's Royal Table (characters vary depending on the meal).
However, if it were up to me, I'd hop on the resort monorail and head over to the Grand Floridian for a character meal at 1900 Park Fare for dinner. As always characters are subject to change but you may run into Cinderella, Prince Charming, Lady Tremaine, Anastasia, Drizella and the Fairy Godmother. Not only is the food amazing, but the decorations at the Grand Floridian are beyond amazing!
Remember, you can make your dining reservations 90 days prior to your check-in date. I'd recommending securing your reservation early to get the time/place you most want.
I hope your daughter has the most magical birthday ever! Diane :)