We usually have dressing room moms assigned to each room, and an ASM (on headset) in charge of fetching kids when they're needed. (The leads are usually pretty responsible for themselves, although there's been times when we've had eight-year-olds wrangling their twelve-year-old costars without being asked... This system is mostly for the chorus kids.) We have dressing rooms near enough to the stage that kids are not allowed out of them until the scene before they are needed, then there is a sort of holding area (where the stairs to the pit are) before the wings. The holding area has tile floor, and the wings have wood, so we tell them to get off the wood but no talking once they're out of the dressing room. If they whisper in the holding area, it's not a huge deal, which is why we keep them there.
In the dressing rooms, they are allowed to talk, bring homework/games/books/etc., as long as it doesn't get too loud. If it's a show where they have huge chunks of time between scenes, the room mom for that room often asks permission to bring a portable DVD player and a movie, and as long as the mom is ready to pause the DVD as soon as the ASM in charge of the kids says they're needed.
I've found one of my problems with noisy kids is that the mothers insist their child needs help with costume changes (and perhaps the kid does), but then insists on staying in the dressing room, undermining the mom who has been assigned to that room, and sending the kids out to me when they aren't needed. Then the kid comes out and talks, and must be sent back to their room and reprimanded for obeying their parents. No matter how many times the director addresses this, it doesn't change, and we can't ban the moms from the dressing rooms because if we did then the kids wouldn't get changed in time.