I would do it on a non EMH day. The performance is the same but you can save yourself a lot of waiting time.
Be there as early as possible for rope drop and go towards tomorrowland. Head for the Launch Bay and the sign up with be on the left side where the exit from launch bay is located. The kids are processed on a first come first serve basis but because they spend a bit of time "interviewing" your kids to see if they can pay attention and not cause a problem, the line can get big quick and take a long time to process. I recommend getting a fast pass for a ride and then go directly to it, or just head straight for it (which is what I did). After you are processed you then get to pick which show you want to be in, so you can pick a later one if you want to. You need to arrive about thirty minutes before the show back where you processed the kids, NOT the stage. They will then take the kids and get them dressed and it gives you a free twenty minutes to catch your breath. They will take all the parents over to the show (it is a little disorganized so pay attention) and they have an area of the floor for you to sit at the front to watch your kids. Please be mindful of the instructions and realize they have a hard time trying to herd a bunch of sugar fed kids through the process.
Your kid will fight either Darth Vader or the female Inquisitor character, but not both. You don't get to choose and they switch the lines back and forth so you won't know which one it will be. You are able to move around if you don't have a good angle for photos, but I found the middle is best.
