Magento – How to show all category and their subcategory in menu in magento2

Try this:

$collection = Mage::getResourceModel('catalog/category_collection')
    ->addAttributeToSelect('*')
    ->addAttributeToFilter('is_active', 1) //only active categories
    ->addAttributeToFilter('include_in_menu', 0) //only categories not included in menu
    ->addAttributeToFilter('parent_id', 10)//get only subcategories of the category with id 10
    ->addAttributeToSort('position')//sort by position
;

foreach ($collection as $category) {
    //do something with $category
}

To do this you'll first want to (if you haven't already) create a custom template directory within the RWD design package. This avoids re or overwriting your default templates that live in magento/app/design/frontend/rwd/default/template.

For the sake of example we'll create magento/app/design/frontend/rwd/custom/template

The specific template we're going to be editing is the topmenu renderer.phtml - which resides at magento/app/design/frontend/rwd/default/template/page/html/topmenu/renderer.phtml. To extend this file properly, create a matching directory structure within the magento/app/design/frontend/rwd/custom/template directory we just created - you should end up with a directory that looks like this: magento/app/design/frontend/rwd/custom/template/page/html/topmenu

Once your topmenu template directory has been created, copy the renderer.phtml file from rwd/default/template/page/html/topmenu into the rwd/custom/template/page/html/topmenu directory you just created.

This file should contain the following code at ~ lines 62 - 71:

    if (!empty($_hasChildren)) {
    $html .= '<ul class="level'. $childLevel .'">';
    $html .=     '<li class="level'. $nextChildLevel .'">';
    $html .=         '<a class="level'. $nextChildLevel .'" href="'. $child->getUrl() .'">';
    $html .=             $this->__('View All ') . $this->escapeHtml($this->__($child->getName()));
    $html .=         '</a>';
    $html .=     '</li>';
    $html .=     $this->render($child, $childrenWrapClass);
    $html .= '</ul>';
}

In your copied file, you'll want to remove or comment out the mid-section so that you're left with:

    if (!empty($_hasChildren)) {
    $html .= '<ul class="level'. $childLevel .'">';
    $html .=     $this->render($child, $childrenWrapClass);
    $html .= '</ul>';
}

Once you've saved your file you can go into adminhtml -> System -> Configuration -> General -> Design -> Themes and set the field "Templates" to the value "custom" -> Save Config and then clear cache.

You should now no longer see the "View All XXX" portion of your sites navigation!