Get current post’s child page?

Question

$ancestors = array();
$ancestors = get_ancestors($post->ID,'page');
$parent = (!empty($ancestors)) ? array_pop($ancestors) : $post->ID;

$parent should be the topmost page parent.

However, if you are using a WordPress generated menu there is a pretty good chance that there is already a item identified by a CSS class as the parent. Take a good look at the generated markup.

http://codex.wordpress.org/Function_Reference/wp_nav_menu#Menu_Item_CSS_Classes

Note: using wp_list_pages, for nested pages, current_page_parent applies to the immediate parent only but current_page_ancestor is applied to all of the parent pages. That is the class you want (I think). Just adjust your CSS.