Reports

Perhaps you can do this with :target^mdn instead of with javascript?

section:not(:target) :not(:first-child) { display: none; }

<section id="parent1">
  <h1><a href="#parent1">Parent1</a></h1>
  <ul class="child">
    <li>
      Some content
    </li>
  </ul>
</section>
<section id="parent2">
  <h1><a href="#parent2">Parent2</a></h1>
  <ul class="child">
    <li>
      Some content
    </li>
  </ul>
</section>
<section id="parent3">
  <h1><a href="#parent3">Parent3</a></h1>
  <ul class="child">
    <li>
      Some content
    </li>
  </ul>
</section>

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