Can't stop nested movie clips

1

I’m a AS3 newbie and I’m having trouble understanding how to access nested movie clips. I want to stop 3 movies, 1 parent and 2 child movies nested inside it. I can stop the parent but I can’t access the child movies with any success. Here’s what i’ve tried…

this.contentMC.stop(); //this works
this.contentMC.avatarMC.stop();//this generates error below
this.contentMC.avatarMC.headMC.stop();();//this generates error below

Error #1009: Cannot access a property or method of a null object reference.

I also tried using MovieClip()

this.contentMC.stop();
MovieClip(parent).avatarMC.stop();
MovieClip(parent.parent).headMC.stop();

But this generates Error #1034: Type Coercion failed: cannot convert flash.display

Can someone please shed some light on this? I searched the forums but couldn’t find anything to help, so I apologize in advance if this has been answered before.

Thanks!