addChild doesn't behave as wanted

I’m new to AS3. I’m trying to make a basic menu which loads MC’s into another MC.
My problem is that when using addChild it just add the MC into the Container (called “Content”) and doesn’t remove existing added children. Looks weird.

Here is my code so far: http://pastie.org/339392

If you have any other suggestion, besides a solution to my problem, everything is VERY welcome as I would like to learn good coding :slight_smile: