[css] How to set opacity in parent div and not affect in child div?

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

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