UPDATE 11/15/10: Please note that the process for signing up has changed, at least for now. You can read more about it here.
Is your kid a Star Wars fan? At Disney Hollywood Studios your little Padawan can confront Darth Vader and train to be a Jedi in Jedi Training Academy (JTA). Located in the back of the park right next to Star Tours, JTA is a 30-minute live show where kids learn lightsaber basics and then take turns confronting the big guy himself, Darth Vader. The show, which loosely follows a script, is funny and moves quickly. While there are inside jokes for Star Wars fans, you don't have to follow the movies to enjoy it. There's no specific age range, but most kids are between 4 and 12 years old. Right now, show times are as follows: 9:55, 10:50 and 11:45am, 12:40, 1:30, 2:20, 4:00 and 4:45pm . This varies throughout the year, so check a website like Steve Soares' for show times before you plan your day.
In the past, children were chosen radomly, often the loudest, most enthusiastic kids or, at least, the kids with the loudest most enthusiastic parents. This led to a lot of disappointed little Padawans. Now if you want to be part of JTA, you wait. Groups of 15 (sometimes more during busier times) are taken onstage and trained . The show, as noted above, is 30 minutes. Quite often, there is a 30 minute or longer wait between shows, which means you could wait through several shows and intermissions before your child gets a chance. There are two "holding areas" where kids wait. The first is the line for the for the attraction itself, which gets longer as the day progresses. When their turn comes up, kids are taken in groups of 15 into the second holding area right next to the stage where cast members ask their ages and if they're afraid of Darth Vader.
[For an update on how children may now participate in Jedi Training Academy, please see this post.]
The new system works really well in the sense that any kid who is willing to wait will get a chance, but it does mean longer waits. Your best bet is to get to Hollywood Studios when the park opens. In fact, given how long the lines are for Toy Story Midway Mania (TSMM), this is one day you'll want to make sure to get to the park at rope drop. When the park opens, send a member of your party over to TSMM for fastpasses if you plan on visiting that attraction. The rest of you, including those who want to participate in Jedi Training Academy, will go and get in line. Don't worry about trying to be first in line for your TSMM fastpasses; you actually want a later return time, say 11:30 or so, giving you enough time to participate in JTA, have a bathroom break, and head over to the other side of the park.
A couple of last thoughts. There's very little that's scary in JTA, but Darth Vader in full regalia can be intimidating for some children, even those in the audience. The show isn't as excessively loud as some shows are at Disney World, so kids with sensory issues should be fine with the noise level, but let your child's personality be your guide. In addition, as all Star Wars fans know, the color of your lightsaber is very important and if your kid is a big fan, he or she probably has a preferred color when playing at home. Let him know ahead of time that he won't be able to choose his lightsaber color. It sounds like a little thing, but I've seen younger kids really upset by not getting "their" lightsaber color. Finally, there's no shade next to the stage, so if you have a long wait, remember to keep hydrated and reapply sunblock.
And of course, may the force be with you!