Magento 1.9 Category – How to Get All Children Categories of a Specific Category

Try replacing this line:

$catlistHtml = getTreeCategories($rootCatId, false);

with this one:

$catlistHtml = getTreeCategories(7, false);

You can use getChildren to get child category data in your block even Use Flat Catalog Category is enabled.

For example

namespace Custom\Module\Block\World;
class Index extends \Magento\Framework\View\Element\Template
{
    protected $_categoryFactory;
    protected $_category;
    protected $_categoryHelper;
    protected $_categoryRepository;

    public function __construct(
        \Magento\Backend\Block\Template\Context $context,        
        \Magento\Catalog\Model\CategoryFactory $categoryFactory,
        \Magento\Catalog\Helper\Category $categoryHelper,
        \Magento\Catalog\Model\CategoryRepository $categoryRepository,        
        array $data = []
    )
    {
        $this->_categoryFactory = $categoryFactory;        
        parent::__construct($context, $data);
    }
        /**
     * Get category object
     * Using $_categoryFactory
     *
     * @return \Magento\Catalog\Model\Category
     */
    public function getCategory($categoryId) 
    {
        $this->_category = $this->_categoryFactory->create();
        $this->_category->load($categoryId);        
        return $this->_category;
    }


    /**
     * Retrieve children ids comma separated
     *
     * @return string
     */
    public function getChildren($categoryId = false)
    {
        if ($this->_category) {
            return $this->_category->getChildren();
        } else {
            return $this->getCategory($categoryId)->getChildren();
        }        
    }  



}

Inside the phtml template.

$test =  $block->getChildren('20');
echo "<pre>";
print_r($test);

Out put

Reference link for more details.

Recommended to pass classes to constructor as a dependency.

Hope it helps.