Hi Cara -
There are rides and attractions appropriate for preschool and young elementary age children at all of Disney's theme parks and water parks.
A great resource for you is the Walt Disney World website "Little Ones at Disney Theme Parks" page found
HERE. If you look at the right side of that page, you'll see a link called "Attraction Height Requirements." Based on the height of your children, you'll be able to see here which attractions they will be able to experience. You'll also find there travel tips and park maps with rides highlighted that are best for younger children.
When my children were the age of yours, one of their favorite activities was meeting characters in the parks and at meals. As for rides, they enjoyed Peter Pan's Flight, Small World, the Tea Cups, Buzz Lightyear, Tomorrowland Speedway, Dumbo, Aladdin's Carpets, Winnie the Pooh, the Carousel, and much more - and that's just at the Magic Kingdom.
Have a wonderful vacation!