Create a simple Dropdown (specific category)

I want to create a dropdown menu where my readers can easily choose posts by Brands, reviews etc.

I used this one, but i dont’ want all categories to show up. Only the childs of Brands (in this case the category are named “varumarken”).

?? I’m glad if someone can help me!

<li id="categories"><h2><?php _e('S?k via Varum?rke'); ?></h2>
	<?php wp_dropdown_categories('show_option_none=Select category'); ?>

<script type="text/javascript"><!--
    var dropdown = document.getElementById("cat");
    function onCatChange() {
		if ( dropdown.options[dropdown.selectedIndex].value > 0 ) {
			location.href = "<?php echo get_option('home');
?>/?cat="+dropdown.options[dropdown.selectedIndex].value;
		}
    }
    dropdown.onchange = onCatChange;
--></script>
</li>