Here's my two cents:
Now, I live in FL, so FEB can be FREEZING to me-pack everything from shorts and t's to coats, gloves, hats. Don't forget the swimsuits. I would pack at least two shirts per day (we like to take breaks and change for the evening). Extra shoes is a good idea.
If you are staying a week or two and you don't mind doing laundry (I do mind), I would build that time into your schedule and bring your own detergent, dryer sheets, and quarters. Maybe some bags to carry it in. Brign a good book to read while you are doing the laundry.
If you have a portable DVD player (or can borrow one), we find that this is excellent for the trip and you can hook it up to the tv in the room. Get the kids some new movies just for the trip. We carry them in a CD carrying case. Our son also has one of those new video game things (his is Spongebob) that hooks right up to the tv. The games are contained in the joystick itself. They are around 14.99 and can be found in the toy dept of Target, K mart ,etc. Believe me, there will be times that you want some down time in your room.
Also, pack medications!!! I practically pack my whole medicine cabinet. The best way to keep things to a minimum is to buy pill/capsule/chewable versions of the meds and bring them in a large ziploc (don't forget the moleskin-I just cut it up into different sized pieces). I also pack a few of each med in a small ziploc and one of us carries it in our fanny packs. Believe me, it happens. Once my son had an earache in the MK. It did us NO GOOD that his tylenol was back in the room! Very important: Take antibacterial gel into the parks and use it!!!!! We also bring a small ziploc with wipes and dental care gum-it allows for "cleanup" after a meal and you do feel a lot better.
Also, in your fanny packs- don't bring your whole wallet-just your id, cc, cash, insurance cards-the basics.
I always pack a nightlight, a pair of scissors, and tape. Also, make sure the kids have your cell # on them and that they know what to do if they get lost.
One item that might seem VERY strange is a humidifier (ours is small). It helps when you are running the heat in the room, if someone gets a cold, and to block out noise. One night at Pop Century, we thought we were going to PASS OUT because the room was SOOOOOO DRY. We actually opened the door and had our door open in the middle of the night (we were desperate).
Have fun!