What is a difference in rendering process for different types of blocks in Magento?
Mage_Core_Block_Text : This is a simple text block so it is not usual that it has child. If so, they will not be automatically rendered, and you would need to render it manually in code.
Mage_Core_Block_Template : Childs of template based block renders when calling getChildHtml from template file. The block should be defined as child either by layout directive or by code. But they are not automatically rendered.
Mage_Core_Block_Text_List : All the childs of a block of this type are automatically rendered when toHtml is called.