ActionScript 3.0 :: Access To A Child Not Working?

I am adding a movieclip in a class I will call class B that is added to a class I will call class A. In class B I am adding the movieclip like this:

var thumbHolder:MovieClip = new MovieClip();
thumbHolder.name = "thumb" + loadingCount;
MovieClip(root).slideMenuHolder_mc.slideMenu.thumbs_mc.addChild(thumbHolder);
trace("after: thumbHolder.name is: " + thumbHolder.name);

In class A, I can do something like this:
thumbs_mc.alpha = .3;
 
But if I try to do something like this, I get an error, even though the trace above traces out the names correctly. thumbs_mc.thumb1.alpha = .3;