Convert Magento 1 PHTML Code to Magento 2

I have a template from a customer’s Magento 1 theme that I need to move to M2. The theme isn’t the same but the template does what I need it to on M1 and I need this in Magento 2.

    

        < ?php 
        $parentCategoryId = 20;
        $cat = Mage::getModel('catalog/category')->load($parentCategoryId);
        $subcats = $cat->getChildren(); 
        foreach(explode(',',$subcats) as $subCatid){
            $_category = Mage::getModel('catalog/category')->load($subCatid);
            if($_category->getIsActive()) {
                $image = Mage::getBaseUrl('media') . 'catalog/category/' . $_category->getThumbnail();
                $imageCheck = $_category->getThumbnail();
                $name = $_category->getName();
                echo '
';
                    echo '';
                        if ($imageCheck == ''){ $image = $this->getSkinUrl('images/noPic.jpg'); }
                        echo '';
                        echo '
' . $name . '
';
                    echo '';
                echo '
';
            }
        }
        ?>
    

This is PHTML from M1, which I did not write. In effect, it loads all subcategories of a parent category then displays them in a grid with the category title and thumbnail. If I have to create a small extension then call what I need in a PHTML, that’s fine but I’m still a little new to M2 so I’m not sure where to start.

I did find this answer and I’m using the top bit with the object manager: https://magento.stackexchange.com/a/201951/69191

I know that’s probably not the best practice but is it passable in a template file, or is there a better way?