Wow! You probably could get by with one good-sized checked bag (26") and one carry-on bag.
Wear one pair of walking shoes. They require too much space in your luggage to pack them.
Colour-coordinate your clothing and accessories to minimize the number of pieces you need for each person.
Carry-on: Complete change of clothing + extra set of undies for each person; nightwear for each person, swimsuit (if planning to swim), 1 pair lightweight sandals/flip-flops per person (can be used for swimming, around the resort and/or slippers) and any medications/contact lenses, glasses, etc. Look for toiletries in 1 oz or small containers (I usually purchase empty containers and fill them with my favourite products--my skin is very sensitive and even a week can wreak havoc with it!). Hairbrush/comb for each person. One curling iron or flat iron, if needed. Minimize jewelry--keep it plain and simple with one or two back-up items only. If you use a windbreaker type jacket, you probably can scrunch it inside the bag, as well, without adding bulk or weight.
Checked bag: 3 bottoms and 5 tops each, with 1 long-sleeved button-front shirt to use as a jacket or overshirt. Undies for 5 days (you're wearing one set and packed 2 in the carry-on) for each person. 1 pair of walking shoes per person (you're each wearing one pair). 6 pairs of socks (if wearing sneakers or similar shoes) per person. If necessary, one slightly dressier pair of slacks/capris and top per person (i.e., Signature Dining). Additional small-sized toiletries (deodorant, lotion, shaving supplies, etc.). Laundry detergent (enough for two loads--you can buy some at WDW, if necessary).
Plan to do laundry on Day 4. You should still have one clean set of clothing each at that point.
Probably forgot an item or two, but you get the idea. You don't need daily outfits that don't coordinate with one another and shoes/accessories to go with each. It's nice, but also can be expensive when flying these days!
We usually visit WDW in December, and as others have noted, have to be prepared for fairly extreme fluctuations in weather--by our standards! Eighty degrees is comfortable, but forty degrees is cold! *LOL*
It really is possible to visit WDW with reasonably minimal luggage!
