I agree with the double stroller idea. It's a lot of walking for a 5 yo and you won't want to go that slowly. If it's crowded, it will be harder to hold on to them. Here's my suggestions for packing light:
If you plan to take a break mid-day (I suggest you do), then only pack for half the day and re-load before you go out again in the late afternoon. The less, the better! On wdwig.com there is a great section on packing for the parks. If you stay onsite, condense your cards down by having your passes and charging priveleges on your room key. Don't bring any extra cards with you (just id and med. ins cards), some cash, wear watches, rain ponchos (the dollar store ones that fold real small-leave them in their packaging until you need them), a travel size sunscreen and travel size wipes . I would get a bunch of the wipes in the soft packaging(not the hard plastic), a couple of diapers, and a t-shirt or tank top for each child. I would bring sunhats for the kids with the brim allaround , and a couple of bandaids. You can bring a sippy cup for the baby with juice and then just fill it up with cold water from the fountains after that. Bring a couple of water bottle for yourselves and the 5yo. Don't forget your camera, autograph book and pen. Try to distribute these items into fannypacks or backpacks , so that there isn't one huge bag.