CustomUIC.as

package {

	import flash.display.Sprite;
	import flash.events.MouseEvent;

	import mx.controls.Alert;

	import mx.core.UIComponent;
	import mx.events.FlexEvent;

	public class CustomUIC extends UIComponent {
		public function CustomUIC() {
			this.addEventListener(FlexEvent.CREATION_COMPLETE, creationCompleteHandler);
			this.addEventListener(MouseEvent.CLICK, clickHandler)
		}

		private function creationCompleteHandler(event:FlexEvent):void {
			this.graphics.beginFill(0x333333);
			this.graphics.drawRect(0, 0, 400, 400);
			this.graphics.endFill();

			this.buttonMode = true;

			var childBox:Sprite = new Sprite();
			childBox.graphics.beginFill(0xFFFFFF);
			childBox.graphics.drawRect(15, 15, 370, 370);
			childBox.graphics.endFill();
			childBox.mouseEnabled = false; // <-- without this it screws up the hit area
			addChild(childBox);

			var hit:Sprite = new Sprite();
			hit.graphics.beginFill(0xCC0000);
			hit.graphics.drawRect(40, 40, 320, 320);
			hit.graphics.endFill();
			addChild(hit);

			hitArea = hit;
			//hit.visible = false;
			hit.mouseEnabled = false;
		}

		private function clickHandler(event:MouseEvent):void {
			Alert.show("You clicked me");
		}
	}
}

this.mouseChildren = false; on the UIC works as well.