Considering your situation, I'm guessing you haven't stayed at a deluxe yet. I would go for the deluxe upgrade with fewer days in your case. But, considering the age of your kids, might I suggest a monorail resort. More specifically the Poly, where you can swim after dark while looking at the MK castle (while swimming). You can watch the water pageant, and the MK fireworks from your Resort. Also, the rooms are on the large side, and room refurbishments are going on right now, so within a few months all the Poly rooms will be redone. I have walked throughout the whole resort, and their Garden View rooms (which are the lowest cost room category) have nice views. Even those that are off the parking lots have nice views.
Only bad thing about this is you'll always want to go back and it is expensive. But, I think it would be worth it.
One more thought is shortening one more day (to 4 days) and upgrading to club level. I know many may think this is a crazy suggestion, but with your family makeup I think club level would make things extremely convenient. Well, now you are talking about a very short trip but you'll have memories that will last a life time, and isn't that what taking family vacations is all about - memories.