Hi Anna,
Currently there is not a Character Dining experience in which you can see favorites from Toy Story. My son was a huge Buzz Lightyear fan, so I always wished there was one too.
The good news is that your little Toy Story fans can still spend time visiting with Buzz, Woody and Jessie in the Magic Kingdom. Buzz can often be found near the exit of his
Buzz Lightyear's Space Ranger Spin in Tomorrowland. Woody and Jessie often greet guests in Frontierland near Splash Mountain. Your best bet is to pick up a Show Times Guide when entering the Park. Meet and Greet locations and times will be listed on there. If there aren't any Toy Story characters listed, be sure to check with Guest Relations. They can help you locate them if any are making appearances that day (in any of the Parks).
In addition, you can often find Buzz, Woody & Lotso at Disney's Hollywood Studios in Pixar Place across from Toy Story Mania. In addition, there are often Green Army Men in the streets of Pixar Place. The characters are also often in the various parades.
Hope this helps and that you have a wonderful vacation,
Kaylene