Hi Michael,
There is plenty to do for children (and adults) of all ages at Walt Disney World. I started bringing my own children to the parks when they were under two years old and we always felt like we had more than enough to do.
The ride participation is solely based on height, not age. Your littlest one can go on any ride for which there is no height requirement. This gives you many dozens of choices including Buzz Lightyear Space Ranger Spin, It's A Small World, Winnie the Pooh, Dumbo, Mad Tea Cups, Carousel, Mickey's Philharmagic, Peter Pan, Pirates of the Caribbean, Country Bear Jamboree, Haunted Mansion, character greetings, character meals, parades, and much, much more (and that's just at the Magic Kingdom).
Your seven year old will be able to do all that, plus more of the "big kid" rides. Depending on his or her height, this might include Space Mountain, Splash Mountain, and Big Thunder Mountain, as well as the thrill rides at the other parks. Check the
Disney attractions page for more information about height requirements.
Good luck with your planning. Please let us know if you have additional questions.