css - How to use vertical align in bootstrap

Question

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>

UPDATE

Some CSS:

.col-lg-12{
top:40px;
bottom:0px;
border:4px solid green;


}

  .child1a, .child1b{
  border:4px solid black;
  display:inline-block !important;
}

.child1{
  border:4px solid blue;
  height:300px;
  display:table-cell !important;
  vertical-align:middle;
}

.child2{
  border:4px solid red;
}

Answer 1

.parent {
    display: table;
    table-layout: fixed;
}

.child {
    display:table-cell;
    vertical-align:middle;
    text-align:center;
}

table-layout: fixed prevents breaking the functionality of the col-* classes.