wp_nav_menu custom walker class to add li-class for children

Hi everybody,

I extended the wp-nav-menu walker class and need to get a custom class into the children list.
I assume it must be coded into the start_el function starting at line 67:
https://phpxref.ftwr.co.uk/wordpress/nav.html?wp-includes/nav-menu-template.php.source.html
I copied it to my functions.php

I managed to get the “topnav” added to the parent list but failed with the nested ul subnav list-class.
it should look like this:

<ul id="navigation">
<li><a href="#">Home</a></li>
<li class="topnav"><a href="#">Subs</a>
   <ul>
   <li class="subnav"><a href="#">Sub-Link 1</a></li>
   <li class="subnav"><a href="#">Sub-Link 2</a></li>
   </ul>
</li>
</ul>

thanks for every help.