<div>
<h3>
    <a href="">
        <?php
        $categoryId = 65;
        $_objectManager = MagentoFrameworkAppObjectManager::getInstance();
        $category = $_objectManager->create('MagentoCatalogModelCategory')
            ->load($categoryId);
        $childrenCategories = $category->getChildrenCategories();
        echo $category->getName();
        ?>
    </a>
</h3>
<ul>
    <?php foreach($childrenCategories as $childrenCategory) {
            echo "<li>" . "<a href=" . $childrenCategory->getUrl() . ">" .  $childrenCategory->getName() . "</a>" . "</li>";}
        ?>
</ul>

{{block class="VendorModuleBlockBlockName" category_id="49" template="Vendor_Module::storecategories.phtml"}}