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:

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