Hi Erin,
How exciting to be planning your kids' first Disney World vacation!
The first place I like to check when planning our vacation is
www.touringplans.com. This site uses a number of factors like past attendance numbers, holidays and special events to determine an expected crowd level for each day. The site also projects what the best and worst days of the week are to visit the four parks. Based on their information, the best days to visit the Disney World parks are Magic Kingdom: Monday, Epcot: Wednesday, Animal Kingdom: Friday and Hollywood Studios: Saturday. While this information is not an exact science, it is a good place to start when planning. I have also found that the parks that offer Extra Magic Hours tends to be very crowded. If you are earlier risers and want to enjoy the extra park time, plan on being at the park for the very start of EMH and then park hop over to another park when the crowds are increasing.
Given your children's ages, I would focus on Magic Kingdom and Hollywood Studios. Hollywood Studios offers Playhouse Disney as well as a great Honey I Shrunk the Kids playground. There is also a funny Muppets 3D show there as well as a beautiful Voyage of the Little Mermaid and Beauty and the Beast Stage Show. Magic Kingdom offers the largest number of attractions your family can enjoy together and numerous meet and greet experiences with the Disney Pals and Princesses. Be sure to spend plenty of time in Fantasyland and don;t forget to stop by Toontown and sneak a peek at Mickey and Minnie's house. Donald has also docked his boat, but it has sprung a leak, so it is a great place for the kids to cool off if needed.
Be sure to have the kids' autograph books ready. I have found the act of interacting with the characters for an autograph often helps ease a young child's initial fears. Don't forget to ask for the kids' First Time Visitor buttons. Have a magical and amazing time!