Problem:

The issue here is that the user wanted to delete their child posts once the parent is deleted on the frontend.

Solution:
This can be done by using the hook below.

add_action( 'before_delete_post', 'ts_before_delete_post', 1, 2 );
function ts_before_delete_post( $post_id, $post ){
  
    if ( $post->post_type == 'persoon' && $post->post_status == 'publish' )
    {       
        $children = toolset_get_related_posts( $post_id,'relatie_personen_phvs',array('query_by_role' => 'parent','role_to_return' => 'child'));
  
        if ( is_array( $children ) )
        {
            foreach ($children as $key => $child) {
                wp_delete_post( $child, true );
            }
        }
    }
}

Add it to your Toolset custom code settings in Toolset->Settings->Custom Code. Ensure that you have activated it.

Next you will need to replace 'persoon' with the Parent Post type and "relatie_personen_phvs" with the post relationship slug.

This will ensure that the child posts are deleted when the parent is deleted.

Finally this will only work if the action of the toolset delete button is set to 'delete'. This will not work when it is set to 'trash'