Code :

<div class="exp-col-content-holder"><a class="expand-cnt-link" href="#">Parent</a>
<div class="hidden-content">
<div class="exp-col-content-holder"><a class="expand-cnt-link" href="#">Child 1</a>
<div class="hidden-content">
<div class="exp-col-content-holder"><a class="expand-cnt-link" href="#">Child of child 1</a>
<div class="hidden-content">Child of child 1 content</div>
</div>
</div>
</div>
<div class="exp-col-content-holder"><a class="expand-cnt-link" href="#">Child 2</a>
<div class="hidden-content">
<div class="exp-col-content-holder"><a class="expand-cnt-link" href="#">Child 1 of child 2</a>
<div class="hidden-content">Child 1 of child 2 content</div>
</div>
<div class="exp-col-content-holder"><a class="expand-cnt-link" href="#">Child 2 of child 2</a>
<div class="hidden-content">Child 2 of child 2 content</div>
</div>
<div class="exp-col-content-holder"><a class="expand-cnt-link" href="#">Child 3 of child 2</a>
<div class="hidden-content">Child 3 of child 2 content</div>
</div>
</div>
</div>
<div class="exp-col-content-holder"><a class="expand-cnt-link" href="#">Child 3</a>
<div class="hidden-content">Child 3 content</div>
</div>
</div>
</div>

Result :

Source code :