AFAIK, there's no built in event to tell when a movie is playing or stopped, surprisingly. However, you are in control of when the main movie is stopped, so you could use a recursive function stop all children and sub children:
ActionScript Code:
function stopAllChildren(container:DisplayObjectContainer){
var i:int = container.numChildren, child:DisplayObject, mc:MovieClip;
while(i--){
child = container.getChildAt(i);
if(child is MovieClip){
mc = child as MovieClip;
mc.stop();
if(mc.numChildren)
stopAllChildren(mc);
}
}
}