I worked on a production of Annie last year as the SM and had no ASM, so I could only be in so many places at once and during the productions I moved set pieces and props backstage.
Don't be afraid to ask for a second set of hands to help "babysit." Depending on the amount of time they are actually onstage, having someone to wrangle the kids, help keep the kids quiet and out of the way when they aren't onstage, etc., can be a great help. A few of the mom's volunteered to sit backstage with the kids to keep the quiet during the run of the show. (A word of caution: try not to pick a stage parent or parent who puts a lot of pressure on their kids as it can add to the nervousness...) It was immeasurably helpful to not have to worry about them getting into trouble.
For rehearsals, encourage them to bring books, homework, and other quiet projects for them to work on. Keeping them entertained is key.
The one thing I was not expecting was for 10 year olds to bring cell phones to rehearsal. And no matter how many times I told them that the phones had to be turned off, there were days when phones would ring.... or they would sneak into the back of the theatre and play games (which would inevitably have sound effects)... which got a bit frustrating.
Most of all, be patient. It's easy to get frustrated and lose your cool, but remember that they are kids and just want to have fun. So do what you can to make things fun but keep things professional and rehearsals running smoothly at the same time.