How to use vertical align in bootstrap

How to use vertical align in bootstrap

Simple problem: How do I vertically align a col within a col using
bootstrap? Example here (I want to vertically align child1a and child1b):
http://bootply.com/73666
HTML
<div class="col-lg-12">
<div class="col-lg-6 col-md-6 col-12 child1">
<div class="col-12 child1a">Child content 1a</div>
<div class="col-12 child1b">Child content 1b</div>
</div>
<div class="col-lg-6 col-md-6 col-12 child2">
Child content 2<br>
Child content 2<br>
Child content 2<br>
Child content 2<br>
Child content 2<br>
</div>
</div>