Debug time.
Code:
var d:DisplayObjectContainer = DisplayObjectContainer(map_mc.getChildByName("target_" + mc + "_mc"));
trace("target_"+mc+"_mc is:"+d);

var tmc:MovieClip = d.getChildByName("target_mc");
trace ("target_mc is:"+tmc);

tmc.gotoAndPlay("over");
I'm guessing you will get "target_mc is: null" and then an error.