Wordpress - How to get all children and grandchildren of a hierarchical custom post type?

You will need to loop over those posts and then do more queries for each post, repeating until you find no posts in a query.

e.g.

function get_posts_children($parent_id){
    $children = array();
    // grab the posts children
    $posts = get_posts( array( 'numberposts' => -1, 'post_status' => 'publish', 'post_type' => 'microsite', 'post_parent' => $parent_id, 'suppress_filters' => false ));
    // now grab the grand children
    foreach( $posts as $child ){
        // recursion!! hurrah
        $gchildren = get_posts_children($child->ID);
        // merge the grand children into the children array
        if( !empty($gchildren) ) {
            $children = array_merge($children, $gchildren);
        }
    }
    // merge in the direct descendants we found earlier
    $children = array_merge($children,$posts);
    return $children;
}

// example of using above, lets call it and print out the results
$descendants = get_posts_children($post->ID);
echo '<pre>';
print_r($descendants);
echo '</pre>';

Yes the above function calls itself, it's a recursive function. It will keep calling itself until it reaches down to a point where the post being looked at has no children, then it will return without calling itself, and the whole stack will bubble back up building the array of children. You would do good to do further research in this area.

Note that there is an inherent cost to what you want, regardless of wether you use recursive functions or not, that is tied to how many levels of posts you have. 5 levels of posts will be costlier than 2, and it is not a linear scaling. You may want to use transients to cache your output depending on how you do this.

Another way of reducing the cost is by only looking down the tree of posts a certain number of levels, e.g. grandchildren but no great grandchildren. This can be done by passing in a depth parameter, and decrementing it on each recursive call, making sure to return an empty array at the start if the depth is 0 or lower. Many tutorials on recursive functions use this as an example.