Did you try this? I am not sure if this is what you look for exactly.
$customs = get_posts('numberposts=-1&post_type=page&post_parent=1&meta_key=test_value');
foreach($customs as $custom) {
print get_post_meta($custom->ID, 'test_value', $single = true);
print '<br />';
}
Querying posts that are “pages” and child of ID 1 and returning pages only with custom field “test_value” then prints the value of custom key in a loop.