List the 5 most recent child pages

I created a hierarchical custom post type for a gallery. The parent pages are the galleries and the child pages are the albums. I want to display the most recent albums on my sidebar (so basically the most recent child pages).

'post_parent' => 0

I only find this in the codex, but this is the opposite i want, this will only list the parent pages. Here is my code so far:

$albums = new WP_Query(array(
    'post_type' => 'gallery',
    'posts_per_page' => 6
));

Assuming you know (or know how to get) the $id (as an integer) of the parent post, use the post_parent parameter:

$albums = new WP_Query(array(
    'post_type' => 'gallery',
    'posts_per_page' => 6,
    'post_parent' => $id
));

Edit

Based on this comment:

I don’t know the parent post. I want to list the most recent child pages from ANY parent page.

While it is easy to query only parent pages (by passing 'post_parent' => 0), it is not quite so simple to query only child pages.

One method would be to query all gallery posts, loop through them, and add any that have a post_parent into another variable; then output that variable.

// Query all gallery posts
$all_albums = new WP_Query( array(
    'post_type' => 'gallery',
    'posts_per_page' => -1
) );

// Array to hold latest albums
$latest_child_albums = array();
// Latest album counter
$latest_albums_count = 0;
// If we already have 5, no need to keep looping
while ( 5 < $latest_albums_count ) {
    // Loop through each gallery post
    foreach ( $all_albums as $album ) {
        // If the current post has a parent
        if ( $album->post_parent != 0 ) {
            // Add it to the latest albums array
            $latest_child_albums[] = $album;
            // Increment the counter
            $latest_albums_count++;
        }
    }
}

// Loop through the array of latest albums
if ( $latest_child_albums->have_posts() ) : while ( $latest_child_albums->have_posts() ) : $latest_child_albums->the_post();
    // Loop content here
endwhile; endif;
// Be kind; rewind.
wp_reset_postdata();