rightTemplate

rightTemplate.php

/* modx event OnHandleRequest  */

<?php
if (isset($_GET['a'])) {
      $action = $modx->getObject('modAction', $_GET['a']);
  	if (is_object($action) && $action->get('controller') == 'resource/create') {
  		$parentID = isset($_REQUEST['parent']) ? (int) $_REQUEST['parent'] : 0; 
  		if ($parent = $modx->getObject('modResource', $parentID)) {
			$parentTpl = $parent->get('template');
			if ($parentTplObj = $modx->getObject('modTemplate', $parentTpl)) {
				if ($props = $parentTplObj->getProperties()) {
        			$tpl = ($ff = $modx->fromJSON($props['childTemplate'])) ? $ff : null;
					if ($tpl) {
						$_GET['template'] = $tpl;
					}
				}
			}
  		}	
	}
}

Is depreciated. Another option https://gist.github.com/bfncs/1474540