Cant find the Child

programming flash
1 
public function playGame2():void
        {
            screens1.switchTo("introScreen");
            addChild(screens1);
            trace(screens1.numChildren);
        }

if the above is my code i get 1

public function playGame2():void
        {
            screens1.switchTo("introScreen");
            addChild(screens1);
            trace(screens1.screenlayer.numChildren);
        }

if the above is my code i get 1

public function playGame2():void
         {
             screens1.switchTo("introScreen");
             addChild(screens1);
             trace(screens1.screenlayer.introScreen.numChildren);
         }

if the above is my code i get :

TypeError: Error #1010: A term is undefined and has no properties.
at Document/playGame2()
at Document/playGame()
at Document/onCompletelyDownloaded()

and introScreen has a button which can be pressed and tries even to dispatch an event though noone can hear it?

What am I missing?