First of all, sorry for my English
When I try to show the class unread in sub-categories this is what happens ..

Categories in the tpl file:

<!-- BEGIN categories -->
<div class="col-md-1 hidden-sm hidden-xs stats">
  <span class="{../unread-class} human-readable-number" title="{../totalPostCount}">{../totalPostCount}</span><br />
  <small>[[global:posts]]</small>
</div>
<!-- END categories -->

Print:

<div class="col-md-1 hidden-sm hidden-xs stats">
  <span class="unread human-readable-number" title="1">1</span><br>
  <small>Posts</small>
</div>

Sub-categories in the tpl file:

<!-- BEGIN children -->
<div class="col-md-1 hidden-sm hidden-xs stats">
  <span class="{../unread-class} human-readable-number" title="{../totalPostCount}">{../totalPostCount}</span><br />
  <small>[[global:posts]]</small>
</div>
<!-- END children -->

Print:

<div class="col-md-1 hidden-sm hidden-xs stats">
  <span class="{children.unread-class} human-readable-number" title="10">10</span><br>
  <small>Post</small>
</div>

In this forum the same thing happens.
Sub-categories:
http://image.prntscr.com/image/be1c184f84954e65968d021b3a89b729.png

Categories:
http://image.prntscr.com/image/0f7726d11dba4827aeb118b464b6ce8a.png