ActionScript 3.0 :: Targeting Code Placed Movieclips?

I am still sorting out how things translate to AS3 so here it goes:
 
In the land of AS2:
 
Say you for looped 5 times and placed movieclips on the stage (attachMovieClip) each one having an mc inside called box_mc. You named them in the loop mc_1, mc_2, mc_3, mc_4, mc_5.Then later you wanted to rotate that box in them you could for loop again and target/rotate the box with a line like this:

this["mc_"+i].box_mc.rotation +=5;

Now in the land of AS3:I place 5 movieclips using addChild and the way I later target the box_mc inside them is to use two lines something like this:

[code]...

I know this method works, but is this the most efficient/correct way of doing this? I tried to get to the box_mc level when assigning the targetMc var, but never found a way that didn't error.Compounding question, how do you make an old line of AS2 work in the AS3 world like this:

this["mc_"+i]["child_mc_"+j].box_mc.rotation +=5;