[Molecula]

Сдвигать надо относительно контейнера.

square_mc = this.createEmptyMovieClip("square_mc", this.getNextHighestDepth());
square_mc.beginFill(0xFF0000);
square_mc.moveTo(10, 10);
square_mc.lineTo(100, 10);
square_mc.lineTo(100, 100);
square_mc.lineTo(10, 100);
square_mc.lineTo(10, 10);
square_mc.endFill();
trace(square_mc._width)
trace(square_mc._height)
ball = square_mc.createEmptyMovieClip("ball", this.getNextHighestDepth());
ball.beginFill(0x00FF00);
ball.moveTo(10, 10);
ball.lineTo(100, 10);
ball.lineTo(100, 100);
ball.lineTo(10, 100);
ball.lineTo(10, 10);
ball.endFill();
trace(ball._x)
trace(ball._y)
ball._x-=ball._parent._width/2;
ball._y-=ball._parent._height/2; 
trace(ball._x)
trace(ball._y)