如果选中，则显示/隐藏UL子项

将.children设置为display:none;。

则从.parent中查找第一个复选框。如果选中，则添加change事件，然后添加toggle .children。

为了对所有列表更加通用，请使用this，返回.parent并找到要切换的.children。

$('.parent input[type="checkbox"]:first-child').change(function(){
    $(this).closest('.parent').find('.children').toggle($(this).is(':checked'));
});

$('.parent input[type="checkbox"]:first-child').change(function() {
  $(this).closest('.parent').find('.children').toggle($(this).is(':checked'));
});

.children {
  display: none;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="refine-categories">
  <h4>Categories</h4>
  <ul class="parent cf">
    <li id='project_category-3' class="popular-category">
      <label class="selectit">
        <input value="3" type="checkbox" name="cat_project_category[]" id="in-project_category-3" />Aanbouw &amp; Opbouw</label>
      <ul class='children'>
        <li id='project_category-33'>
          <label class="selectit">
            <input value="33" type="checkbox" name="cat_project_category[]" id="in-project_category-33" />aanbouw</label>
        </li>
        <li id='project_category-34'>
          <label class="selectit">
            <input value="34" type="checkbox" name="cat_project_category[]" id="in-project_category-34" />algemeen</label>
        </li>
        <li id='project_category-35'>
          <label class="selectit">
            <input value="35" type="checkbox" name="cat_project_category[]" id="in-project_category-35" />Fundering en heiwerken</label>
        </li>
        <li id='project_category-36'>
          <label class="selectit">
            <input value="36" type="checkbox" name="cat_project_category[]" id="in-project_category-36" />garage bouwen</label>
        </li>
        <li id='project_category-37'>
          <label class="selectit">
            <input value="37" type="checkbox" name="cat_project_category[]" id="in-project_category-37" />isoleren</label>
        </li>
        <li id='project_category-38'>
          <label class="selectit">
            <input value="38" type="checkbox" name="cat_project_category[]" id="in-project_category-38" />nieuwbouw</label>
        </li>
        <li id='project_category-39'>
          <label class="selectit">
            <input value="39" type="checkbox" name="cat_project_category[]" id="in-project_category-39" />opbouw</label>
        </li>
        <li id='project_category-40'>
          <label class="selectit">
            <input value="40" type="checkbox" name="cat_project_category[]" id="in-project_category-40" />renovatie</label>
        </li>
        <li id='project_category-41'>
          <label class="selectit">
            <input value="41" type="checkbox" name="cat_project_category[]" id="in-project_category-41" />veranda en serre</label>
        </li>
    </li>
    </ul>
</div>
<div id="refine-categories">
  <h4>Categories 2</h4>
  <ul class="parent cf">
    <li id='project_category-3' class="popular-category">
      <label class="selectit">
        <input value="3" type="checkbox" name="cat_project_category[]" id="in-project_category-3" />Aanbouw &amp; Opbouw</label>
      <ul class='children'>
        <li id='project_category-33'>
          <label class="selectit">
            <input value="33" type="checkbox" name="cat_project_category[]" id="in-project_category-33" />aanbouw</label>
        </li>
        <li id='project_category-34'>
          <label class="selectit">
            <input value="34" type="checkbox" name="cat_project_category[]" id="in-project_category-34" />algemeen</label>
        </li>
        <li id='project_category-35'>
          <label class="selectit">
            <input value="35" type="checkbox" name="cat_project_category[]" id="in-project_category-35" />Fundering en heiwerken</label>
        </li>
        <li id='project_category-36'>
          <label class="selectit">
            <input value="36" type="checkbox" name="cat_project_category[]" id="in-project_category-36" />garage bouwen</label>
        </li>
        <li id='project_category-37'>
          <label class="selectit">
            <input value="37" type="checkbox" name="cat_project_category[]" id="in-project_category-37" />isoleren</label>
        </li>
        <li id='project_category-38'>
          <label class="selectit">
            <input value="38" type="checkbox" name="cat_project_category[]" id="in-project_category-38" />nieuwbouw</label>
        </li>
        <li id='project_category-39'>
          <label class="selectit">
            <input value="39" type="checkbox" name="cat_project_category[]" id="in-project_category-39" />opbouw</label>
        </li>
        <li id='project_category-40'>
          <label class="selectit">
            <input value="40" type="checkbox" name="cat_project_category[]" id="in-project_category-40" />renovatie</label>
        </li>
        <li id='project_category-41'>
          <label class="selectit">
            <input value="41" type="checkbox" name="cat_project_category[]" id="in-project_category-41" />veranda en serre</label>
        </li>
    </li>
    </ul>
</div>

您需要为此使用一些JQuery。

$('#in-project_category-3').click(function () {
    $(".children").toggle(this.checked);
});

而您的孩子ul需要获得display: none

我给你做了一把小提琴，我希望这是你想要的：https://jsfiddle.net/jfuhdq0g/2/