It's my understanding that you can't really "surprise" your kid with this- in that your kid has to be in line with you when you sign him/her up. So you can't go add him to the list and then later take him to the show and say "surprise! You're in it!"
I know that 4 is the youngest they allow; how does this little experience actually work? We've got 2 days planned in DHS and plan to show up and just watch one of the sessions with our daughter (who turns 4 on the first day of our trip) to see if she's interested in it or if she's scared off. If she's interested, then on our second day in that park we plan to sign her up. When it's time for your session, do you get to go with your kid the whole time? Do they take them "backstage" or whatever without you? (I've never seen this in person, just
YouTube videos, so pardon my ignorance.) I feel like she might need Mommy within line-of-sight for moral support, so this may make-or-break her decision to try it.