Custom menu – Child menus

I have a custom menu in a wordpress website.

To display the menu I use

<?php wp_nav_menu( array('menu' => 'side_nav' )); ?>

And the CSS looks like this:

[CSS moderated as per the Forum Rules. Please just post a link to your site.]

OK, looks good.

Link 1
SubLink1
SubLink2
Link 2
SubLink3
SubLink4

(You get it)

What I want to do is so when the parent (Link1/Link2) is CLICKED (not hovered) but clicked, so the “parent” link is active, only then will the sublinks to THAT “parent” show.

How do I do this?

eg.

Link 1 (active/clicked/on that page)
SubLink1
Sublink2
Link 2
Link 3