Keep parent page active

I have a parent page on my main menu. When you click on that page, you are presented a sidebar with another menu listing of all of its child pages. When you click on one of the child pages, the parent page is no longer active in the top menu.

How do I make both the parent page active on top and the active child page active in my left menu?

Thanks

The parent page li receives the css classes current_page_parent and current_page_ancestor, style them to be similar to, or add them to the rules for current_page_item or current-menu-item in your theme’s style.css.