JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="MainNavi_clone" class="menu"><li id="menu-item-660" class="menu-item menu-item-type-custom menu-item-object-custom has_children  publish" data-item='{"id":"660","has_children":true,"has_content":false}'><a href="/spielen/" class="no-rgba" data-slug="">Spielen</a>
<ul class="sub-menu di_1" data-depthindex="1">
    <li id="menu-item-661" class="menu-item  menu-item-object-spielen publish" data-item='{"id":"37","has_children":false,"has_content":true}'><a href="http://foederalion.bundesrat.de.babiel.com/spielen/das-stimmenspiel/" class="no-rgba" data-slug="">Das Stimmenspiel</a></li>
    <li id="menu-item-662" class="menu-item  menu-item-object-spielen publish" data-item='{"id":"36","has_children":false,"has_content":true}'><a href="http://foederalion.bundesrat.de.babiel.com/spielen/foederalion-das-raetsel-unter-dem-eis/" class="no-rgba" data-slug="">Föderalion – Das Rätsel unter dem Eis</a>    <li id="menu-item-663" class="menu-item  menu-item-object-spielen publish" data-item='{"id":"33","has_children":false,"has_content":true}'><a href="http://foederalion.bundesrat.de.babiel.com/spielen/gesetze-durchbringen/" class="no-rgba" data-slug="">Gesetze durchbringen</a>    <li id="menu-item-664" class="menu-item  menu-item-object-spielen publish" data-item='{"id":"35","has_children":false,"has_content":true}'><a href="http://foederalion.bundesrat.de.babiel.com/spielen/wer-sitzt-wo/" class="no-rgba" data-slug="">Wer sitzt wo?</a></ul>

<li id="menu-item-655" class="main menu-item menu-item-type-custom menu-item-object-custom has_children  publish" data-item='{"id":"655","has_children":true,"has_content":false}'><a title="/wissen/" href="/wissen/" class="no-rgba" data-slug="">Wissen</a>
<ul class="sub-menu di_1" data-depthindex="1">
    <li id="menu-item-2070" class="menu-item  menu-item-object-wissen publish" data-item='{"id":"714","has_children":false,"has_content":true}'><a href="http://foederalion.bundesrat.de.babiel.com/wissen/bundesquest/" class="no-rgba"...

JavaScript

tab_id = 4;
clones = $('#MainNavi_clone > li:nth-child(' + (tab_id + 1) + ') li');
//clones = $('#MainNavi_clone > li:eq(' + tab_id + ') li');

clones.css('border', '1px dashed red');