If you do a search on the Tips board or even the attractions board you'll probably find many threads on packing or pack lists. I know there are even some on the web. But since you asked, Here are items from my master packing list--but keep in mind every situation is different. The ages of your children will obviously affect your list (diapers, kid toys, special blankets versus GameBoys & video games!). Also, we went on the cruise after the parks....The time of year will make a difference. We had to pack hats, gloves and jackets.
"Administrative Items:" tickets, reservation numbers, maps, guidebooks, journals, special coupons, travellers' cheques, ID tags for the kids (I put our family picture inside a clear plastic pocket and put critical info on the back such as our cell phone numbers), passports & birth certificates (for cruise), single bills for tipping, cell phones, walkie-talkies, copies of all tickets/documents, pre-printed address labels for post cards (remember to send some to yourselves so you have them waiting at home!), post card stamps EXTRA Large (2gallon) ziplock bags to bag the clothing into sets and use for packing.
"Because Mom said so" items: cheap rain ponchos, belly bags, motion sick medicine, ibuprofen/acetominephen/aspirin, antacids if you use them, ear plugs, band-aids, mole skin (we didn't use it), spare tote or gym bags, ziplock bags for wet rides (we did use these), cameras, film, spare batteries, over-the-door shoe bag (purchased at a dollar store) for toiletry items, etc., dollar store laundry basket (the kind that fold flat), kitchen trash bags, night light, laundry detergent & softener sheets, chargers for cell phones/camcorders, clothespins, corkscrew, travel alarm clock, small spray air freshener
"Fun stuff:" glow necklaces, goodies for kids (snacks, small toys, disposable cameras), GameBoys/games/batteries, sharpies for autographs, deck of cards, reading material, small flashlights/penlights, duct tape (just because), small cooler bag,
"Clothing items:" Don't over pack clothes. You'll buy some there (t-shirts especially). We layered in December (t-shirts, sweatshirts, windbreaker jackets), baseball caps, lots of socks, sneakers & sandals, we had to bring dressy clothes for the cruise but would not have done so for the parks, shorts, pants, (plan ahead on how many days and whether you plan to do laundry), swimsuits, pajamas, belts, undergarments, hats, gloves
"Toiletries:" hair accessories for girls, toothbrush/toothpaste, combs/brushes, razors, contacts/solutions/spares/glasses, prescriptions if any, sunglasses, hand cream, facial cleansers/makeup, lip balm, feminine products if needed, sunscreen
--you asked!