D.W.
You can always buy diapers once you get into town to save space. There is a wal-mart in very close proximity to Disney. Put the kids in tennis shoes. They are going to take them off for any water areas but I'm not remembering any water areas that the regular Disney parks have. Maybe they've added some since I've been there though.
I always plan an outfit a day (maybe 2 if I know we'd be going out to dinner too but on big park days we'd eat either there or right after leaving so no need to change clothes) but carry a back up outfit with me at all times. All the hotels have washer/dryers so you can always toss a load of laundry in if you need to.
Don't forget to take things like the baby tylenol, sunscreen and such!