so I have a div which is min-height 100%; (100% of the screen or more). The child holds the content and needs to be centered vertically, but it might extend the parent to more t
Use the display: table; and display: table-cell; properties.  
The outer DIV will need to have display: table; and the inner DIV display: table-cell; along with vertical-align: middle;.  Now you will be mimicing the default display of a td.
CSS
.parent {
  min-height: 100%;
  display: table;
}
.child {
  display: table-cell;
  vertical-align: middle; 
}
HTML
<div class="parent">
     <div class="child">child</div>
</div>
This question has been asked often. A simple search here on SO or Google will get you plenting of results.