If you simply want to add a child after a certain amount of time, use a Timer object.
Actionscript Code:
public var delay:int = 3000; // 3 seconds
public var repeats:int = 1; // run one time
public var myTimer:Timer = new Timer(delay, repeats);
myTimer.addEventListener(TimerEvent.TIMER, onTimer);
public function onTimer(e:TimerEvent):void{
// do something
}
myTimer.start();
If you need to respond to a tween finishing, use TweenLite's onComplete object prop.
Actionscript Code:
TweenLite.to(myClip, 1, {x:20, y:20, onComplete:myCompleteFunction});
public function myCompleteFunction():void{
// do something
}