Definitely ask them to bring something QUIET to do...homework, quiet games, knitting, coloring books, and so on. If they will not be required to be onstage for extended periods, have a rehearsal hall or other seperate room where they can blow off steam and be a little louder.
Give them a talk outlining your expectations for them...to be quiet, to listen to you and other pertinent adults, to walk, not run, in the theater, to stay in their seats, and so on.
Bring your patience. And sense of humor. Talk to them. If they have a conversation with you about school, music, whatever, you become more of a person to them rather than just "that adult we have to listen to."
I have used parent volunteers with great success. We make it mandatory for each child's parent, guardian, or even older brother/sister (over 18 and pre-approved) to take a shift watching over the holding room. A shift is typically from call time for a certain rehearsal or performance, to the time the show is over. It's maybe a three-hour commitment, total. If you have a large chorus, and everyone's parent goes once, you have at least a couple of weeks covered.
There will be parents who are unable to make it because of work, or other legitimate reasons, and you may have some uncooperative parents who refuse to sign on. But there are usually a few parents who are eager to take on more shifts than one. We usually offer 2 comps for each shift worked (with approval of house management), which makes parents very happy.
Their only duty is to shush the kids if they get too rowdy, and generally be available for the kids if they need small things. The volunteers are not expected to teach or entertain the kids. The kids bring their own entertainment.
As for incentive, if they are mature in attitude, the privilege of performing provides its own reward. I have never used an incentive system because it is one more thing to keep track of, along with cues, reports, etc. The rewards can get very expensive, and parents may not approve of sugary candy or stickers that may migrate to furniture. It also is easy to guess what may motivate a kid, but when you offer it to them, they may not be motivated by it at all. I have had many kids with a surprisingly professional atttitude who need no incentive to behave themselves.
