public function beforeLoadLayout($observer)
{

$fullAction=$observer->getEvent()->getFullActionName()
if($fullAction=='catalog_category_view'):

$currentCategoryId=  Mage::getSingleton('catalog/layer')->getCurrentCategory()->getId();

$children = Mage::getModel('catalog/category')->getCategories($currentCategoryId);
if(count(count($children)>0)){
       $observer->getEvent()->getLayout()->getUpdate()
       ->addHandle('cat_has_child_cat');
}else{
      $observer->getEvent()->getLayout()->getUpdate()
       ->addHandle('cat_has_no_child_cat');
}
endif;
}