You have to call a render function recursive for each subtree. As long as there is another subtree it will output another -list.
main_view.php
<?php echo new View('view2',array('node'=>$root_node)) ?>
view2.php
<ul>
<?php foreach ($node->children() as $child):?>
<li><?php echo $child->title?></li>
<?php if ( ! $child->is_leaf()): ?>
<?php echo new View('view2',array('node'=>$child)) ?>
<?php endif; ?>
<?php endforeach; ?>
<ul>
If you also want your HTML intended correct you can use the $node->level() function. But that is really unnecessary