We went in June at WDW. My suggestion...make SURE they are wearing Star Wars clothing. Encourage them to look cute, beg, wave their arms, etc, when they start picking kids to go onstage. I noticed that, in the crowd we were in, they picked kids who were on the line alone. Those who had parents standing with them, trying to wave the CMs over, tended to get passed by.
The whole thing, including wait time, was about half an hour. Are your kids Star Wars geeks? If so, do not pass this one up. My son has his "Padawan" certificate framed and on his desk. He is so into Star Wars that he will tell anyone who listens that he is now a padawan...and what that entails.
In fact, I already have the "make your own lightsaber" kit from
WalMart in the closet for Christmas, as he read that one thing a padawan has to do prior to being promoted to Jedi is build his own lightsaber. (Someone in marketing deserves a well padded retirement fund for thinking that one up!)
Your kids are about the right age. It seemed that most of the ones picked were around my son's age...he's 7.
My only thought is...would it hurt any of their feelings if they didn't get picked but one of their siblings did? You might prepare them for the possibility.
That said, be sure to move to the front or right of the stage if they do get picked. All our photos of my son dueling Darth Vader were taken from behind Vader...I wish I'd gotten even one profile shot...because nobody in the crowd would let us scootch over to get pics!