get_term_children returns WP_Error for custom taxonomy

Since get_term_link() will return a WP_Error object if the term does not exist, you could try:

$termchildren = get_term_children( $taxID, $taxType );

echo '<ul>';
foreach ( $termchildren as $child ) {
    $term = get_term_by( 'id', $child, $taxType );

    $term_link = get_term_link( $term->name, $taxType );

    if( ! is_wp_error( $term_link ) )    
         echo '<li><a href="' . $term_link . '">' . $term->name . '</a></li>';

}
echo '</ul>';

so it looks like you are trying to echo the WP_Error object.

You could try to debug it with:

if( is_wp_error( $term_link ) )    
    echo $term_link->get_error_message();