Don't worry, I brought my oldest to WDW for the first time when she was almost two and we had a great time.
The vast majority of rides and other attractions at WDW can be experienced by everyone, including toddlers and preschoolers. There is a handy chart
here that lists the rides in each of the theme parks that have height restrictions (for safety). As you will see, the lists are very short. And depending on how tall your older children are, they might also be excluded from some of the rides on the list. Everything else that does not have height restrictions, your youngest can go on with the entire family.
Some favorites of preschoolers include the Fantasyland classics in the Magic Kingdom including: Dumbo, the Carousel, it's a small world, the Many Adventures of Winnie the Pooh, and Peter Pan's Flight. Younger kids also like the Country Bear Jamboree and the Jungle Cruise. In the other parks, some of the favorites of younger kids are: the Playhouse Disney show at Disney's Hollywood Studios, the Kilimanjaro Safari at the Animal Kingdom, and the Nemo ride at Epcot.
In addition to rides, my girls loved watching the parades when they were two. And their most favorite thing of all was meeting the characters. They loved getting hugs from Mickey and all his friends.
I'm sure you and your family will have a great time at WDW.