Add Child dynamically in a loop

Hi,

I am creating new instances from a movie clip in library and adding it to a container on the stage. I also need to add a child to that already added child. Some thing like this in a loop.

var bar:YearBar=new YearBar();
myContainer.addChild(bar);

then I have to do this:
 var cube:Cube=new Cube();

I want to add this to the bar, so I did this:
myContainer.bar.addChild(cube);

But I am getting error.
Please note all these are happening in a loop and I need to create different number of these instances.

What am I doing wrong?