Add dynamic meta value to wp_list_pages

Hi.
I need a list of all of a page’s child pages with custom meta values beside each title.
I have the function for the child pages working just fine:

function wpb_list_child_pages() { 
global $post; 
if ( is_page() && $post->post_parent )
	$childpages = wp_list_pages( 'sort_column=menu_order&title_li=&child_of=' . $post->post_parent . '&echo=0' );
else
	$childpages = wp_list_pages( 'sort_column=menu_order&title_li=&child_of=' . $post->ID . '&echo=0' );
if ( $childpages ) {
	$string = '<ul>' . $childpages . '</ul>';
}
return $string;
}

But I can’t seem to be able to add the meta key to it.
I need the output to be something like:

<ul>
<li>Child page title <span><?php $key="duration"; echo get_post_meta($post->ID, $key, true); ?></span></li>
</ul>

Thoughts?