We've been to WDW a bunch of times with DD between 3.5 and 5.5. My pared-down list:
change of clothes and small handtowel - flattened in a gallon ziploc
quart ziploc with meds/health items - tums, ibuprofen, tiny brush, sunscreen, antibacterial handwash, bandaids, shout wipes, hair clip, lip balm, kleenex
quart ziploc with snacks (no dining plan) - gummie snacks, yogos, granola bars. (I used to take healthier snacks but grapes/apples got smushed/bruised and yogurts I couldn't keep cold)
Autograph book/pen
5 bottles of water and carabeener/strap for holding
camera/camcorder
mini fan (2) - one with a flashlight, one with a mister
hats / sunglasses for all (3)
Minnie ears
glow sticks (saved my bacon one night in a long line to see princesses! we passed them out to everyone near us!)
I've never had to use the spare clothes, towel, or shout wipes - everything else is fine.
I used to carry: jacket/sweatshirt for DD (for a winter trip would probably re-add), games for DD (she's older now and entertained by reading maps and playing with the fans), more snacks/juice boxes (in a cooler bag)