Since you will be at WDW before your cruise you will probably have everything you already need. The kids will probably live in their swim suits by day, chanage for dinner and then back into swim suits or shorts after dinner. If you're going on a 3-4 day cruise don't worry about dressing up. The girls can bring a sun dress and you and the boys can wear docker style pants with golf type collared shirts.
After overpacking on our first cruise we now pack very light.
This is what we take for me DH, DS 18 and DD 14.
For a seven day we each only take one small wheeled carry on suitcase.If more room is needed for things like iPods, cameras or other stuff i let the kids take a small backpack or over the shoulder carry on.
My husband takes a garment bag as his extra carry on with the tuxes and dressy dresses and shoes. We wear the same outfits for both formal and semi formal night and we never feel over dresses.For each of us we take about 3-4 pair of shorts 3-4 tops, seven days of underware, swim suits, and toiletries and something to sleep in. I also take a baggie with Purex detergent sheet and do two loads of laundry haf way through the cruise. Easier to do laundry than lug around an over stuffed suitcase.
This has worked for us. If you over pack you never really wear everything you packed. Have a great time.