Hi,
I have 6 year old twins and we are leaving in 3 weeks too for our 4th trip together as a family (we now also have another little girl who is going to be 4 in dec.)...the first trip we took was when the twins were 14 months and we had a BLAST....my best advice on top of what you already have is:
a. bring plastic bags...Babies R Us has those blue plastic bags in a little round holder thing and sometimes at target you can find them in teeny packs of ten...you just never kno when those babies will come in handy.
b. absolutely spend the money for the disposable bibs, spoons, and sippy cups...so worth it! The sippy cups I would save sometimes and clean them in the room when my supply ran low BUT in general things to take and toss are AWESOME
On the subject of things to throw away, we bought a big box of cheerios and brought a load with us to the parks every day....fabulous decision for snack time and so convenient.
c. have LOTS of wipes...when we took the kids at 14 months it was during food and wine festival and we took the opportunity to let them try all sorts of foods...in short, they are a great quick way to clean dirty/sticky hands and faces.
d. if you aren't staying at a monorail resort but are staying on property rent a car...
e. don't be afraid to take them out for dinner...we took the girls to CA Grill and flying fish and while we were lucky that they behaved themselves, the staff were very accomodating (of course, we took them at like 5pm) and it so wasn't a problem.
f. Food and wine festival is a great win win for every body....you can take your time, there's lots of chances for the babies to walk around and look at interesting things and you and your spouse get to try lots of food and wine... especially if they nap!
g. Take lots and lots of pictures and then take some more. Use the photopass people as much as you can...those are some of the only pics we have with all of us...we were so glad we spent the money to buy the cd and i would recommend it, especially for a family with twins.
IN short, just don't be afraid...we originally went to disney world because we couldn't think of a more child friendly place to vacation and we were absolutely correct!
Have fun!

sss