Generated by SassMeister.com.

SassMeister-input-HTML.html

<ul class="checkbox-tree">
  <li>
    <i class="plus"></i>
    <input id="category" type="checkbox">
    <label for="category">Category</label>
    <ul>
      <li>
        <input type="checkbox">
        <label>Foo</label>
      </li>
      <li>
        <input type="checkbox">
        <label>Bar</label>
      </li>
    </ul>
  </li>
</ul>

SassMeister-input.scss

// ----
// Sass (v3.3.14)
// Compass (v1.0.0.rc.1)
// ----

.plus:after {
  content: '+';
}

.checkbox-tree {
  
  ul {
    display: none;
  }
  
  input:checked ~ ul {
    display: block;
  }
  
  list-style: none;
  
  input {
    //display: none;
  }
  
  
  
}

SassMeister-output.css

.plus:after {
  content: '+';
}

.checkbox-tree {
  list-style: none;
}
.checkbox-tree ul {
  display: none;
}
.checkbox-tree input:checked ~ ul {
  display: block;
}

SassMeister-rendered.html

<ul class="checkbox-tree">
  <li>
    <i class="plus"></i>
    <input id="category" type="checkbox">
    <label for="category">Category</label>
    <ul>
      <li>
        <input type="checkbox">
        <label>Foo</label>
      </li>
      <li>
        <input type="checkbox">
        <label>Bar</label>
      </li>
    </ul>
  </li>
</ul>