<?php
    $parent = get_cat_ID("photos");
    $cats = get_categories('child_of='.$parent);
    foreach ($cats as $cat) {
        echo "<ul>";
        echo sprintf("<li><a href='%s'>%s</a></li>", get_category_link($cat->term_id), apply_filters("get_term", $cat->name));
    echo "</ul>";
} ?>