We travel with one small carry-on wheeled suitcase each, and one personal bag each. Our airline is Continental, so I believe the limit is 21x14x9 (i think). We use a backpack or larger Disney beachbag for each person's personal bag. In those bags we put all of the items we need for in the parks, and any papers we will need. We designate one backpack for everyone's swim gear to put in the lockers by the pool for use when we are done in the park. We also carry one stroller, which they set just inside the plane, and my husband and mom are allowed to carry their C-paps as an extra item for free b/c they are medically necessary. The same goes for my daughters' nebulizer. We are taking a soft sided cooler this year, so that will be my hubby's extra bag. Inside it will be more park stuff, and cameras.
Just some ideas for how to best use the personal bags: In them we put sunscreen, paperwork, change of clothes for any chafing issues or water rides, lysol wipes, sunglasses, batteries, cameras, pin trading lanyards, pressed penny tubes, body glide, changes of shoes for mom, photopasses, etc...Anything we need for that day in the park. That way we can either drop our luggage off in our rooms, load the stroller from the personal bags so we don't have to go through any luggage, and head out to the parks. OR if our rooms aren't ready, we can empty our personal bags into the stroller, drop the swimming gear off into a locker, leave our luggage to be put in the rooms when they are ready, and head out. Not that you asked, but I thought it might help with deciding what size suitcases you can buy. Even if you don't use more than one suitcase for the two boys and you and your hubby, you can always take another bag inside your suitcase to put souvenirs in for on the way home since everyone gets a carry-on.