Hi, Matt!
What a great trip you have planned! Woo hoo!
There are so many ways for your family to meet their favorite Disney characters. For a great overview of where to find them, click on this list of pages entitled "
Character Greetings Around [...] Theme Park." The first four listed are the four main parks at Walt Disney World, and each one will give you specific tips on who you'll find at that particular park.
If you're looking to meet Disney Junior characters, consider booking a reservation for Disney Junior Play 'n Dine, which is held during breakfast and lunch at Disney's Hollywood Studios' Hollywood & Vine Restaurant. At all character greetings, bring your own pen / marker (we like the clickable kind, so that we don't have to worry about losing a cap.) Long ago, we hadn't yet picked up an autograph book before our first character meal, but I had a new small pad of paper and the characters were happy to sign those pages, too. They'll surely sign your DVD inserts, if that's something that you'd prefer. However, you might want to pick up an autograph book per child; they make a great keepsake, and our kids love trying to complete their collections by seeking out characters they haven't yet met. I even pressed each Princess' lipstick print (from my toddler's face) onto the page where each had signed his book. That was 9 years ago, and we still laugh as we glance through that book (he was so smitten!)
Send more questions as you think of them, Matt! We love to help!