You can't. Css today simply doesn't allow that.
The logical rendering model is this one :
If the object is a container element, then the effect is as if the contents of the container element were blended against the current background using a mask where the value of each pixel of the mask is .
Reference : css transparency
The solution is to use a different element composition, usually using fixed or computed positions for what is today defined as a child : it may appear logically and visualy for the user as a child but the element doesn't need to be really a child in your code.
A solution using css : fiddle
.parent {
width:500px;
height:200px;
background-image:url('http://canop.org/blog/wp-content/uploads/2011/11/cropped-bandeau-cr%C3%AAte-011.jpg');
opacity: 0.2;
}
.child {
position: fixed;
top:0;
}
Another solution with javascript : fiddle