Something I didn't realize until our recent trip is that they pick LOTS of kids. Seriously, like 15 - 20 kids must have been up there. I thought they would only have 3 or 4 kids come up front. I didn't know that it was like a huge training class! So there is a good chance your kids will be chosen if they are enthusiastic!
Anyway, if you are there nice and early, that will help. Also, they call up the children based on colours they are wearing, so if you have something nice and bright that stands out, it will help. Like a purple shirt or an orange hat. Many kids were wearing black and white or grey, and I think that makes it harder for the Jedi Master guy to pick them out of the crowd.
There was an adorable little girl, about 9 years old, who had her hair all done up like a Jedi, with a ponytail, and a little braid in the back like Anakin Skywalker. She also was wearing a brown Padowan-type robe, so she TOTALLY got picked to go onstage!
Also, if you get a photopass, you can sit back and enjoy the show. There was a photopass photographer who took awesome pics of all the kids. She gave us a paper with the show time and date on it, which we took to the Photo center to get our photos added to our Photopass. So cool!
Here's a picture of one of my girls fighting Darth Maul:
Good luck!