Hello, Natalie!
Having that first day to rest up is a fantastic idea, especially since you'll be coming such a great distance to get to Walt Disney World.
From your earlier question, I see that you are staying at Disney's Pop Century Resort. I recommend that you spend your first day at the pool, weather permitting, so that you can just relax, swim, and become accustomed to the resort as well as its proximity to the various parks. There are always poolside activities that the kids will love, too, in addition to a playground and arcade as well.
We often like to begin or end our trips to Walt Disney World with a trip to
Downtown Disney, too. An unticketed shopping, dining and entertainment area, you'll be able to go at your own pace, see what you want to and then head back to your resort for some more rest and relaxation. There's even a movie theater where you can even choose to "dine-in." The full-service restaurant Fork & Screen enables you to enjoy the latest movies with your meals (and a cocktail!) delivered right to your seat. The Lego Imagination Center keeps my son busy for quite awhile, as does the build-your-own light saber area at Once Upon a Toy. World of Disney, the world's largest Disney Store, is great for browsing, too. You can make a day at DD as short or as long as you'd like, since the complimentary resort bus runs from Pop Century to DD approximately every 20 minutes.
Send more questions if you'd like help with the rest of your days at Walt Disney World; I'd love to help!