How to force child div to be 100% of parent div's height without specifying parent's height?

For the parent:

display: flex;

You should add some prefixes, http://css-tricks.com/using-flexbox/.

As @Adam Garner noted, align-items: stretch; is not needed. Its usage is also for parent, not children. If you want to define children stretching, you use align-self.

.parent {
  background: red;
  padding: 10px;
  display:flex;
}

.other-child {
  width: 100px;
  background: yellow;
  height: 150px;
  padding: .5rem;
}

.child {  
  width: 100px;
  background: blue;
}

<div class="parent">
  <div class="other-child">
    Only used for stretching the parent
  </div>
  <div class="child"></div>
</div>

NOTE: This answer is applicable to legacy browsers without support for the Flexbox standard. For a modern approach, see: https://stackoverflow.com/a/23300532/1155721

I suggest you take a look at Equal Height Columns with Cross-Browser CSS and No Hacks.

Basically, doing this with CSS in a browser compatible way is not trivial (but trivial with tables) so find yourself an appropriate pre-packaged solution.

Also, the answer varies on whether you want 100% height or equal height. Usually it's equal height. If it's 100% height the answer is slightly different.