SH_Child_Only_Walker.php

class SH_Child_Only_Walker extends Walker_Nav_Menu {

	// Don't start the top level
	function start_lvl(&$output, $depth=0, $args=array()) {
		if( 0 == $depth )
			return;
		parent::start_lvl($output, $depth, $args);
	}

	// Don't end the top level
	function end_lvl(&$output, $depth=0, $args=array()) {
		if( 0 == $depth )
			return;
		parent::end_lvl($output, $depth, $args);
	}

	// Don't print top-level elements
	function start_el(&$output, $item, $depth=0, $args=array()) {
		if( 0 == $depth )
			return;
		parent::start_el($output, $item, $depth, $args);
	}

	function end_el(&$output, $item, $depth=0, $args=array()) {
		if( 0 == $depth )
			return;
		parent::end_el($output, $item, $depth, $args);
	}

	// Only follow down one branch
	function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {

		// Check if element as a 'current element' class
		$current_element_markers = array( 'current-menu-item', 'current-menu-parent', 'current-menu-ancestor' );
		$current_class = array_intersect( $current_element_markers, $element->classes );

		// If element has a 'current' class, it is an ancestor of the current element
		$ancestor_of_current = !empty($current_class);

		// If this is a top-level link and not the current, or ancestor of the current menu item - stop here.
		if ( 0 == $depth && !$ancestor_of_current)
			return;

		parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
	}
}