SilverStripe Page::$disallowed_children

config.yml

CMSMain:
  extensions:
    - DisallowedChildrenExtension

DisallowedChildrenExtension.php

<?php

/**
 * Implements "disallowed children" functionality for page classes. Usage:
 * private static $disallowed_children = array(
 * 	'SomePageType',
 * 	'SomeOtherPageType'
 * );
 * Note: not fully tested (e.g. does this prevent changing parent in "Settings" tab?)
 * @author Loz Calver <kinglozzer@gmail.com>
 */
class DisallowedChildrenExtension extends Extension {

	/**
	 * @param array &$def
	 */
	public function updateSiteTreeHints(array &$def) {
		if(empty($def)) continue;

		foreach($def as $class => $data) {
			if(in_array($class, array('All', 'Root'))) continue;

			$singleton = singleton($class);
			$disallowed = $singleton->stat('disallowed_children');
			if(isset($disallowed) && ! empty($disallowed)) {
				$orig = isset($data['disallowedChildren']) ? $data['disallowedChildren'] : array();

				$disallowed = array_unique(array_merge($orig, $disallowed));
				$def[$class]['disallowedChildren'] = array_values($disallowed);
			}
		}
	}

}