span in shortcut title in .php

Hi,
I would like if it is possible tu use <span> in title of the accordion lines?
for now when I use span in title there is a massage:
“Please enter a title attribute”
(without <span> </span> it is working good. but then I can not change the color of numbers.
Thank You!

What I do is:

/*$content = '[accordion]';
foreach ($child_posts as $child_post) {
	$title = '<span class="nr">'.$child_post->fields['nr'].'.</span> '.$child_post->post_title;

$content .= '[accordion-item title="'.$title.'"]'.$child_post->fields['text'].'[/accordion-item]';
}
$content .= '[/accordion]';
echo do_shortcode($content);*/

So the result should be like
01 Title
02 Title2

https://wordpress.org/plugins/accordion-shortcodes/