1. Luggage
Luggage restrictions and carry-on rules vary from airline to airline. A 21" rolling suitcase may look small in your home, but it'll look huge when you have to put it into an airplane overhead. My own carry-on (for laptop and camera gear) is 19"x13"x9" and I consider it to be the max; any bigger than 19" and you have to turn it sideways to fit in the overhead. Besides, those 21" monsters will get HEAVY - and you will have to pick them up many times as you board and deplane, and you will have to lift them into the overhead.
I say, check the rollers and carry on only your backpacks. If you fly Southwest, checked bags are free anyway, but if you're flying an airline that charges for checked bags, it will be worth the extra fees to not deal with two large carry-on items per person, especially if your kids are small and you have to wrestle that stuff yourself.
2. Snacks
Solid, pre-packaged snacks are okay in a carry on - granola bars or pretzels are the best. But forget any liquids - no juice boxes, no water bottles, etc - because of TSA security regs. Any liquids you want to take aboard with you will have to be purchased in the airport AFTER you go through security; I typically buy a bottle of water for my flight at Hudson News; you can also get sodas and juices. You might also consider eating a meal at the airport food court before bording; this way you won't be hungry during the flight.
3. I guess I have a typical guy's view of clothing; I don't pack "outfits", I pack two pair of pants (1 long, 1 short in December, 2 short during hot months); one shirt for each day of the trip (no extras because I always buy a few Disney t-shirts while there), 1 set of underwear and socks for each day plus one extra, and if I'm going in December I pack 1 sweatshirt. No spare shoes - they're just dead weight. That's in addition to the clothes I wear on the plane, which are typically long pants and a polo shirt; I like to dress nicer when I fly, it makes them think I'm a business traveller, and I seem to get a little better treatment because of that (so long as I treat the airline and airport and TSA folks nice, that is!)
4. The countdown in your signature currently says 11 months, 3 weeks, and 2 days, meaning that you'll be going in mid-August. FORGET THE COLD WEATHER GEAR! WDW is hot in August, and unbelievably humid. It's also a bit wet; take rain ponchos, but forget the sweatshirts and jackets. They'll be dead weight.