Snacks, candy (after all they will be busy the entire trip), glow sticks/bracelets/etc... for night time, if you are flying-an activity book for the plane home, well the drive home too, Coins for pressed pennies if they are into that, Disney money.
As far as the Keens, do they wear them all the time? I would hate for them to get blisters on their first time wearing them. My younger 2 have/had chunkier feet so they always got an irritation from wearing them. I have no issues and can wear a new pair with out problems.
New PJ's. That is what my kids got last year when we were in FL. I told them to only pack one pair and they got a 2nd pair for easter.
My kids get a new Disney hat and the characters sign the hat vs a book this way all I have to carry is a Sharpie.
Potato head pieces, a resort mug (I am chuckling because it may be included or it may not be but they don't need to know that). Socks, slippers, underwear (Stuff they will wear while you are there).
A new outfit, pack one or two less for them.
Hand sanitizer/wipes
Chap stick or a safe alternative for them
Hair ties
tooth brush/paste