How do I add a child to an HTML Tag render element?
Please see the drupal API page on this.

A simple example could be:

// Shows how #prefix and #suffix can add markup into an array.
t('Using #prefix and #suffix') =&gt; array(
  '#markup' =&gt; '&lt;div class="child"&gt;&lt;/div&gt;&lt;div class="child"&gt;&lt;/div&gt;',
  '#prefix' =&gt; '&lt;div id="parent"&gt;',
  '#suffix' =&gt; '&lt;/div&gt;',
),


or you could use children

t('using children demonstration') =&gt; array(
  '#markup' =&gt; '',
  'child1' =&gt; array('#markup' =&gt; '&lt;div class="child"&gt;&lt;/div&gt;'),
  'child2' =&gt; array('#markup' =&gt; '&lt;div class="child"&gt;&lt;/div&gt;'),
  '#prefix' =&gt; '&lt;div id="parent"&gt;',
  '#suffix' =&gt; '&lt;/div&gt;',
),