I want to add the 10 most recent posts under a parent category on the main menu

If I understand you correctly you want the latest posts to appear inside the menu as sub menu items, is that correct? Something like this:

Category One        top menu item
| Post A            sub menu item
| Post B            sub menu item

I have found this solution published here: https://alex.blog/2011/09/20/code-snippet-add-a-link-to-latest-post-to-wordpress-nav-menu/, and changed it so it not only replaces the URL of a menu item but also the title:

// Front end only, don't hack on the settings page
if ( ! is_admin() ) {
    // Hook in early to modify the menu
    // This is before the CSS "selected" classes are calculated
    add_filter( 'wp_get_nav_menu_items', 'replace_placeholder_nav_menu_item_with_latest_post', 10, 3 );
}
// Replaces a custom URL placeholder with the URL to the latest post
function replace_placeholder_nav_menu_item_with_latest_post( $items, $menu, $args ) {
    // Loop through the menu items looking for placeholder(s)
    foreach ( $items as $item ) {
        // Is this the placeholder we're looking for?
        if ( '#latestpost' != $item->url )
            continue;
        // Get the latest post
        $latestpost = get_posts( array(
            'numberposts' => 1,
        ) );
        if ( empty( $latestpost ) )
            continue;
        // Replace the placeholder with the real URL
        $item->url = get_permalink( $latestpost[0]->ID );
        // And also replace the title
        $item->title = $latestpost[0]->post_title;
    }
    // Return the modified (or maybe unmodified) menu items array
    return $items;
}

One would only need to loop through all placeholders #latestpost1, #latestpost2, and so on, and also change $latestpost to get the last, second last, third last of a category.