We were there in September and two of my three children were picked at the first show of the day.
We arrived early, so they were in the front. I made simple signs for them ("The force is strong with this one", with an arrow pointing down.) My oldest didn't want to hold his sign, but the other two did. He was not picked.
Also my middle son had on a birthday pin. He was actually the first kid picked. I think it had to do with his age (8), his Star Wars t-shirt, his birthday pin and his sign. He also has red hair and tends to get noticed.
I was surprised that my daughter (6) was also picked, so I think it had to do with the sign.
I also noticed at that particular show that all the kids with birthday buttons were picked.
Now, a few days later we went back and tried to get my oldest son picked. He agreed to the sign this time and we even put the birthday button on him.
It was a different Jedi master and he didn't choose him. My son decided he didn't want to try again, so we didn't go back.