Another option is to set your child div to display: inline-block;
.content {
display: inline-block;
height: 100%;
width: 100%;
background-color: blue;
}
.container {
display: table;
}
.child {
width: 30px;
background-color: red;
display: table-cell;
vertical-align: top;
}
.content {
display: inline-block;
height: 100%;
width: 100%;
background-color: blue;
}
<div class="container">
<div class="child">
a
<br />a
<br />a
</div>
<div class="child">
a
<br />a
<br />a
<br />a
<br />a
<br />a
<br />a
</div>
<div class="child">
<div class="content">
a
<br />a
<br />a
</div>
</div>
</div>