Hi, MaryJane! The theme parks are not divided up into sections by age, although there are areas that may seem more toddler friendly. You may find, though, that your little one can do many of the same things as your older kids!
I have girls ranging in age from 5-19, and they have always done things like the Haunted Mansion, Jungle Cruise, Pirates of the Caribbean, and Buzz Lightyear together. They also all enjoy the Safari at Disney's Animal Kingdom, Toy Story Mania at Disney's Hollywood Studios, and the Living with the Land boat ride at EPCOT.
If there is an attraction that you want to all do together, there is a child swap option available, which allows one grown up to stay with the little one while the other one rides with the other kids, and then when the ride is over, you switch off. The other kids love this, as it usually means two rides in a row for them without having to stand in line a second time!
You'll find a quick list of a few of the things you might do with your toddler by visiting
HERE.
Have a wonderful vacation!