The easiest way to allow your younger children to get on stage at Disney is at High School Musical. During various parts of each show, they pick about fifteen kids to join the song and dance part of the live event. The whole thing is very well done on an outdoor stage that makes it easy to see it all. Kids love a pep rally, especially by the cool cast of High School Musical.
Some of the best things that happen here on your day at disney are the unexpected. Over the years a lot of things have changed, and more just involvement is a big part of the show it's so entertaining when the kids Get on Stage, and a thrill for the parents watching them. It happens all over the place, just about every show has volunteers from the audience they get to be in the show itself. Somewhere along the way the cast members will ask for volunteers, and you better raise your hand as high up in the air as humanly possible. It probably helps if you're dressed in a pair of Mickey Mouse Ears. I don't think I would hurt your chances of getting out on the stage at all. I love how they take somebody from the audience in the beginning of the Indiana Jones Show comedy. Every things about interactivity, even the Dance Gig for the Children.