Why Doesn't This Work... Targetting An MC Inside MC

[tomyknoker]

Inside my mc btn1 I have another mc called 'arrows' but when I target it nothing happens, it's meant to fade _alpha to 0... Any ideas?

for (i=0; i<numOfBtn+1; i++) {
	this["btn"+i].num = i;
this["btn"+i].onRollOver = function() {
		this.play();
		this.arrows.alphaTo("0", .5, "easeOutQuad", 0);
	};