Hello Barbara! Welcome to the Disney Parks Moms Panel.
  While most families are keen to dress up for Magic Kingdom during 
Mickey's Not-So-Scary Halloween Party, Guests visiting for 
Mickey's Very Merry Christmas Party are welcome to deck themselves in holiday cheer. 
 As far as pajamas are concerned, as long as they adhere to the 
dress code for Walt Disney World Resort Theme Parks than the more the merrier. With the weather being brisker in Central Florida in November and December, be sure that the clothing you choose is warm enough for all the Guests in your group. There is nothing that can dampen your festive spirits more than a cold chill. 
 It sounds as if you are not looking to wear inappropriate attire, but are simply interested in being as comfortable as possible. If that is the case, then pajamas should be fine. 
 Be sure to get 
Disney PhotoPass Photos taken during the party in your festive pajamas with a few Mickey's Very Merry Christmas Party Magic Shot exclusives. My daughter loves the additions to our family photos and the exclusive Special Event Magic Shots are some of the best. 
 Have a Very Merry Visit! 
 ~Todd