We were there on 4/17/2011 the Sunday of Easter break, we didn't get to the park until around 9:50 am because we were getting off a cruise, DHS opened at 8:00 am that day, I went right to TSM grabbed FPs and my wife and kids went right to JTA and were able to sign up for the 11:30 show, but we had ADRs at SciFi so we asked for the 1:30 show.
Our kids loved it - It was probably their favorite expierence all week long. We also forked over about 30 dollars in light sabers after the show because they loved it so much!
They actually had about 30 kids ages 4-12, 15 of them were on the stage fighting Darth Vader and 15 of them were on the ground fighting Darth Maul. Darth Maul was worth the price of admission to DHS - he was a fantastic actor. My kids both faught Darth Maul. The littler kids seemed to be up with Darth Vader. The older kids with darth Maul finished earlier and there was time for one extra kid - so my eldest got to fight twice! She was very happy about that. It was a great show, even if you didn't have a kid in it, because the Jedi master was funny.
So go when you can and you might get lucky and get a spot, the line to sign up is behind the stage. Also I think if you watch a show and a kid doesn't show up sometimes they take a kid. We didn't have any extra spots at our show - but one parent tried to put their kid over the rope, they had to tell the kid to go back (was a little sad, but the kid was fine with it.)