If it does not work, then it is incorrect. But since you didn't provide any details about HOW it doesn't work, I have to guess.

I guess that it's complaining that there is no target_mc property on DisplayObject. You need to cast to MovieClip in order to access dynamic properties. Or, you can use getChildByName again if that's more appropriate.

Code:
MovieClip(map_mc.getChildByName("target_" + mc + "_mc")).target_mc.gotoAndPlay("over");
Or
Code:
MovieClip(DisplayObjectContainer(map_mc.getChildByName("target_" + mc + "_mc")).getChildByname("target_mc")).gotoAndPlay("over");