Magento – Add child to a layout block programmatically

You can add a method on the parent block to fetch the child depending on the product type (I've seen this kind of logic a couple of times in core or something similar):

class ParentBlock 
{
    public function getIntuitiveNameChild($item)
    {
        return $this->getChild("intuitive_child")
                    ->setProductType($item->getProductType()) 
                    // You can also decide the product type in this setter, in the Child block.
                    ->setItem($item);
    }

    public function getIntuitiveNameChildDinamically($item)
    {
        return $this->getChild("intuitive_child_" . $item->getProductType())
                    ->setItem($item); 
    }    
}

// parent tpl
// i suggest you avoid getChildHtml(), unless you're certain that methods won't need to be called from the tpl
echo $this->getIntuitiveNameChild($_item)
          // ->someOtherMethod()
          ->toHtml();

Still, seeing how you modify the layout xml to add children blocks, you may be interested in how Magento decided to work with rendering markup depending on product types in Mage_Sales_Block_Items_Abstract::getItemHtml() and Mage_Checkout_Block_Cart_Abstract::getItemHtml().

So the answer is that it should not be an observed event. Here is how I got it to work, in the config.xml I needed to update the adminhtml node.

<?xml version="1.0" encoding="utf-8"?>
<config>
  <modules>
    <Wsu_NewtworkSecurities>
      <version>0.1.0</version>
    </Wsu_NewtworkSecurities>
  </modules>
  <adminhtml>
    <layout>
      <updates>
        <wsu_newtworksecurities>
          <file>newtworksecurities.xml</file>
        </wsu_newtworksecurities>
      </updates>
    </layout>
  </adminhtml>
</config>

and in

app\design\adminhtml\default\default\layout\wsu\newtworksecurities.xml

<?xml version="1.0"?>
<layout version="0.1.0">
    <feildset>
        <reference name="head">
            <action method="addJs">
                <script>wsu/newtworksecurities/newtworksecurities.js</script>
            </action>
        </reference>
    </feildset>

    <adminhtml_index_login>
        <update handle="feildset" />
        <reference name="form.additional.info">
            <block type="wsu_newtworksecurities/feildset" name="newtworksecurities.feildset"
                   template="wsu/newtworksecurities/feildset.phtml" />
        </reference>
    </adminhtml_index_login>

    <wsu_newtworksecurities_error_index>
        <reference name="content">
            <block type="core/template" name="newtworksecurities.error" template="wsu/newtworksecurities/error.phtml"/>
        </reference>
    </wsu_newtworksecurities_error_index>

</layout>