Can't get getChildCategories to work when looping through parents

Trying to print out a two tier navigation menu of my categories. For each parent category I would like to print out a list of it's child categories. All the demos I see use Mage::get开发者_运维技巧Model, but trying to get it to work with getChildCategories. Check out the code below, the areas commented out are what is breaking it. Any help would be great.

$nl            = chr(10);

$obj         = new Mage_Catalog_Block_Navigation();
$main_cats    = $obj->getStoreCategories();

echo '<ul>'; 

foreach ($main_cats as $main) {

//    $sub_cats     = $this->getChildCategories($main);
    $main_class = ($this->isCategoryActive($main)) ? 'current' : '';

    echo '<li class="'.$main_class.'"><a href="'.$this->getCategoryUrl($main).'">'.$main->getName().'</a>'.$nl;
/*    
    if ($sub_cats->count())) {

        echo '<ul>';
        foreach ($sub_cats as $sub) {
            $sub_class = ($this->isCategoryActive($sub)) ? 'current' : '';
            echo '<li class="'.$sub_class.'"><a href="'.$this->getCategoryUrl($sub).'">'.$sub->getName().'</a></li>'.$nl;
        }
        echo '</ul>'.$nl;
    }
*/    
    echo '</li>';

}

echo '</ul>';

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What do you think about this?

getting_and_using_categories_and_subcategories