Default sidebar, page navigation menu, I want to highlight the item

I need some customization of the sidebar on my site asiabc.co

I leave the sidebar widget empty, so the page navigation menu is present by default.

When user is staying on a parent page, this sidebar menu shows a block of grey color background covering the parent and its children pages. (screen capture: parent.jpg).

However, when the user is staying on its child pages, this highlight background is gone. (screen capture: child.jpg)

I really hope that the same highlight can be present when a user is staying on the parent page as well as its children page for consistency.

Any comments welcome. Thank you very much.