Show/Hide UL child if checked

I'm trying to show/hide some ul checkboxes when the parent is clicked. I've searched everywhere and I just can't get it to work!

Could someone please help me to get this to work? If a parent category is checked, sub checkboxes needs to show/hide.

This is the code:

<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>    

When I click on a parent checkbox I can see the span class is changed to <span class="custom checkbox checked"></span>

Hope someone can help me to sort this out!

Thank you for your time!

PS: You can check the website here: http://bit.ly/20Miaom It's in the sidebar (right).