Make child pages appear in menu when parent page is clicked?

Hi,

My site has a lot of pages. I have been using a plugin called Hierarchical pages, which works very nice but the load on the site is very high. I am using WP's built-in menu's now but the result is not the same. The way I have it now, under 'Afzonderlijke werken' (the 3rd left menu widget) is that child pages are not displayed, or, under 'Kritieken' (the 5th left menu widget) are permanently displayed (because I added them manually to the menu). The idea, however, is to have the menu widgets to display child pages only when the respective parent page link in the menu is clicked. Is there any way to achieve this, please?

Thanks,
Rutger