Dynamic dropdown menus with wp_dropdown_categories

Hi! I have created three dropdown menus with three levels of categories, as seen on:

http://stilius.se/wilink/.

What I need to do is to get only the children of the chosen category in the next menu. For example, if i choose "Örebro", I only want the next menu to show the direct childen of "Örebro", that will be "Karlskoga". If i then choose Karlskoga, I want the next menu to show only "Kläder". Then when I search, I would like to see the posts under that category. Here is my code so far:

<form action="<?php echo home_url( '/' ); ?> method="get" id="searchform" >

<?php $LANargs = array(
'show_option_all' => '',
'show_option_none' => '',
'orderby' => 'ID',
'order' => 'ASC',
'show_count' => 0,
'hide_empty' => 1,
'child_of' => 0,
'exclude' => '',
'echo' => 1,
'selected' => 0,
'hierarchical' => 1,
'name' => 'cat',
'id' => '',
'class' => 'postform',
'depth' => 1,
'tab_index' => 0,
'taxonomy' => 'product_category',
'hide_if_empty' => false
); ?>

<?php wp_dropdown_categories( $LANargs ); ?>

<?php $STADargs = array(
'show_option_all' => '',
'show_option_none' => '',
'orderby' => 'ID',
'order' => 'ASC',
'show_count' => 0,
'hide_empty' => 1,
'child_of' => 0,
'exclude' => '',
'echo' => 1,
'selected' => 1,
'hierarchical' => 1,
'name' => 'cat',
'id' => '',
'class' => 'postform',
'depth' => 2,
'tab_index' => 0,
'taxonomy' => 'product_category',
'hide_if_empty' => false
); ?>

<?php wp_dropdown_categories( $STADargs ); ?>

<?php $KATEGORIargs = array(
'show_option_all' => '',
'show_option_none' => '',
'orderby' => 'ID',
'order' => 'ASC',
'show_count' => 0,
'hide_empty' => 1,
'child_of' => 0,
'exclude' => '',
'echo' => 1,
'selected' => 1,
'hierarchical' => 1,
'name' => 'cat',
'id' => '',
'class' => 'postform',
'depth' => 3,
'tab_index' => 0,
'taxonomy' => 'product_category',
'hide_if_empty' => false
); ?>

<?php wp_dropdown_categories( $KATEGORIargs ); ?>

<input type="submit" class="submit" name="submit" id="searchsubmit" value="<?php esc_attr_e( 'Sök', TEMPLATE_DOMAIN ); ?>" />

</form>

Is it maybe possible to do this via the Javascript onSelect function, or use a function in the child_of value? Any point in the right direction would be helpful.

Thanks!

/Stilius