Hi Christine!
How exciting for all of you to be able to plan such a momentous occasion as your daughter's fifth birthday celebration at Disneyland! I can't think of a better place to have a special birthday.
Mickey's Halloween Party is a great addition to the festivities for the family. Costumes are not required for this but I strongly suggest you get into the spirit of things and definitely have costumes to bring. They do not have to be Disney characters but they must be respectable and not have masks over faces. Please look through the
Costume Guidelines to make sure your idea will adhere to what Disney accepts.
My family has been experiencing the Disney Hallloween fun for years. Most of our visits have been to the Walt Disney World resort parks but the parties are basically the same. As I said definitely arrive in costume and try to come up with a theme for the entire family. We have found this more fun then everyone dressing in their own theme. This is such a great way to come together as a family. Some of the ideas we have used are candy bars, popcorn boxes, bowling pins and one bowling ball, soccer balls, football players, cheerleaders, football, and referee, and coke cans and bottles. As you can see this can be quite interesting to see a bunch of bowling pins walking through the park together. You can do something similar using a Disney theme as well or top off a costume with some special Mickey ears for the Disney add on. Let your creativity run and I am sure you will have some great costumes for the party.