Make subcategories dissappear from widget (custom)menu

Hey,

I’m using the TwentyTwelve theme.

I created a custom menu widget which appears in a sidebar on select pages. This custom menu is used to navigate my product catalog on my site, so it features many sub-categories.

Currently all sub-categories are displayed in the menu widget at all time which, as you can imagine, makes my website very chaotic.

I need a simple solution which hides the subcategories unless the parent category is clicked. I tried:

.children
{
display:none;
}

but it had no effect.

Could some one point me in the right direction?