Hi Ashley,
Your little Princess is going to absolutely love the
Bibbidi Bobbidi Boutique. Both locations carry the Snow White dress up shoes, so I wouldn't worry about bringing them with you.
Mickey's Very Merry Christmas Party is an incredibly fun Event, which I highly recommend. However, I wouldn't dress my children up fancy for it. Since it is held in the Magic Kingdom, the kids will be wanting to ride all of the open attractions. In addition, there will be a lot of walking, so you'll want them to be comfortable. Instead, I would opt for a festive shirt or sweater (with Santa, a Christmas Tree, Angel, Mickey in his Holiday best, etc. on it).
Hope this helps and that you have a wonderful vacation,
Kaylene