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>