do this https://rdrama.net/post/18459/marseycapywalking-megathread-for-bugs-and-suggestions/4060401#context

files/assets/js/comments.js

@@ -39,4 +39,12 @@ function collapse_comment(id) {
 	const ta = document.getElementById('markdown-c_'+id);
 	if (ta && !ta.classList.contains('d-none'))
 		expandMarkdown(`c_${id}`)
+
+	if (closed) {
+		const children_count = document.getElementById(`replies-of-c_${id}`).getElementsByClassName('comment-body').length
+		document.getElementById(`children-count-${id}`).innerHTML = ` (${children_count} children)`
+	}
+	else {
+		document.getElementById(`children-count-${id}`).innerHTML = ''
+	}
 };

files/templates/comments.html

@@ -278,6 +278,8 @@
 							{% if c.blackjack_result %}
 								{{c.blackjack_html | safe}}
 							{% endif %}
+
+							<span id="children-count-{{c.id}}" class="vertical-align ml-1"></span>
 						</div>
 
 						{{macros.reports(c, 'comment')}}