Replace the setChildByIndex order

programming flash
1

I have my background added in the first world of my platform based game like this:

else if (lvlArray* == 'ROCKYHILLS') {            
                    var newRockyHills:backRocky = new backRocky;
                    newRockyHills.x = newRockyHills.width / 2;
                    newRockyHills.y = newRockyHills.height / 2;
                    //newRockyHills.scaleX *= 1.2;
                    currentStage.addChild(newRockyHills);
                    currentStage.setChildIndex(newRockyHills, 0);
                }

Next i want to add the new background, so i wrote this:

else if (lvlArray* == 'FIERYPITS') {
                    
                    var newFieryPits:bgFieryPits = new bgFieryPits;
                    newFieryPits.x = newFieryPits.width / 2;
                    newFieryPits.y = newFieryPits.height / 2;
                    //newRockyHills.scaleX *= 1.2;
                    currentStage.addChild(newFieryPits);
                    currentStage.setChildIndex(newFieryPits, 0);
                    currentStage.setChildIndex(newRockyHills, 1);
                }

But it doesn’t work. How can i change the order and put the new Bg on top?