Question

To give an example, how would I find how many pixels down from the parent is child3?

<div class="parent" id="unique"> 
<div class="child1"></div>
<div class="child2"></div>
<div class="child3"></div>
<div class="child4"></div>
</div>

Assume the classes define the width, heights, padding, margins etc

Answer 1

var child=$('.child3');
var parent=$('.parent');

return child.offset().top-parent.offset().top;

Offset defines the element position relative to the document.