I sorta did the same thing 15 years ago, but only 3 kids.
First of all, how old are the kids????
Second
DO NOT MAKE IT A SURPRISE!!!! We did and it was a COMPLETE TRAIN WRECK!!


The kids had never really been anywhere and they were SO overwhelmed and over stimulated that *all* they wanted to do was go back to the hotel and swim. The kids I took were 14, 16 & 18.
I'm going to steal the advice of others and add some of my own!
Consider half day at DTD followed by mini golf.
Depending upon the ages of the kids, touring monorail resorts and watching ELP. We also took the kids to AKL and ate lunch at their CS.
If you are going to do either of these, do them BEFORE visiting MK or they will be a letdown.
Don't carry a bunch of stuff into the park! Travel LIGHT. Bring a case of water and give every kid a bottle and a small snack for their pocket. Refill bottles at water fountains. Split adult CS meals among the kids. Eat breakfast before you come (your already know this). On such a special trip, _I_ would not want to schlep a cooler. Don't mess with ponchos. No one will melt if it rains.
And ENJOY!