I'll reiterate what some others have said:
Timing is everything. You're looking at one of the most expensive times of year; it's also a time when the ship is likely to be filled to capacity.
How many days do your kids get off at Thanksgiving? Mine get three, so if we were to cruise that week, we could miss only two days of school -- and those two days are usually not very um, academically strenuous. Ditto for fall break week. How about January? When do you go back to school? If you have a few days in there, it could work out. While I am against pulling kids out of school for a full week (and I saw that you have some concerns about this), I really think timing is your best way to save money. If you could find an "odd week" in your school's calendar, it could possibly work out for you.
Still on the subject of timing, make your reservations far in advance. You'll find that the cruise prices RISE as the cruise draws nearer.
Do you have a Disney VISA card? If you make your downpayment with one of these, you can often receive $50 in on-board credit. It's not much, but it would help.
Would you likely buy those overpriced $35/person refill soda mugs? Book through AAA and you'll receive them for free as a "gift" from your
travel agent.
Finally, before you write off the cruise as more expensive than WDW parks, consider that EVERYTHING is included in that high price. Your family will enjoy very nice meals -- without bills at the end. You can easily have a great time onboard and on shore without paying for any additional excursions.
I'd suggest that you read the cruise board (on this message forum). The posters there are a wealth of information!