Here are my recommendations based on what we did when we had an "undersized" child:
Splash - Toy Story Woody & Jessie Meet-and-Greet (near Splash Mountain exit), Pirates of the Caribbean, Country Bear Jamboree, The Laughing Place (a playground underneath the Frontierland Station, roughly next to Splash Mountain), or ride the Walt Disney World Railroad's entire circuit from Frontierland
BTMRR - any of the attractions mentioned above, plus Tom Sawyer Island
Soarin' - Living with the Land, a snack at Sunshine Seasons (which also typically has coloring pages and crayons available), The Circle of Life film
Test Track - Innoventions East (or just go over to Character Spot and/or Club Cool), or head into the Mexico Pavilion in World Showcase for the boat ride, a Kidcot Station, or an Agent P's World Showcase adventure there (one of the sign-up areas should be on your way, along the east side of the entry into World Showcase).
Star Tours - Star Tours exit gift shop (Tatooine Traders - plenty to amuse you if you're a Star Wars fan), Jedi Training Academy show, Muppets 3D, "Honey I Shrunk the Kids" playground