Magento 1.9: How to Create a Popup from a Block

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().

I think Fabian has a good point and any heavy lifting of data should be done by a model.

I would like to add to that however that they way Magento works is generally catching the post of a page in the Controller, sorting out the data and then calling models to process that data.

Although this might not strictly be the way MVC dictates it, it is however how Magento does it and for keeping your code readable and logical for the next developer to work on I would like to advice you to follow this example.

Let's take a simple contact form that is stored in the database as an example. We will have a controller and model in this case. I'll just post the controller code below. The index action displays the form but will post to the postAction where all the data will be handled. After catching the data and validating it we just use the model to persist the data to the database.

In my opinion (correct me if I'm wrong) this is the common way how Magento extensions deal with post data from forms.

class [Namespace]_[Module]_indexController extends Mage_Core_Controller_Front_Action
{
    public function indexAction()
    {
        $this->loadLayout()
            ->renderLayout();
    }

    public function postAction()
    {
        $data = $this->getRequest()->getPost();

        // validate the data
        $validated = true;
        if (!Zend_Validate::is($data['name'], 'NotEmpty')) 
        {
            Mage::getSingleton('customer/session')->addError('Please fill out name');
            $validated = false;
        }
        if (!Zend_Validate::is($data['email'], 'NotEmpty')) 
        {
            Mage::getSingleton('customer/session')->addError('Please fill out email');
            $validated = false;
        }
        if (!Zend_Validate::is($data['message'], 'NotEmpty')) 
        {
            Mage::getSingleton('customer/session')->addError('Please fill out message');
            $validated = false;
        }

        // save the data
        if (!$validated)
        {
            $model = Mage::getModel('[model]/someclass');
            $model->setData(array(
                'sender_name' => $data['name'],
                'sender_email' => $data['email'],
                'message' => $data['message'],
            ));

            try 
            {
                $model->save();
            } 
            catch(Exception $e)
            {
                Mage::getSingleton('customer/session')->addError("{$e}");
                $this->_redirect('[module]/index/index');
            }

            Mage::getSingleton('customer/session')->addSuccess('Succesfully saved');
        }

        $this->_redirect('[module]/index/index');
    }
}