show only on certain pages (dynamically!)

Im sorry if ive missed this somewhere, but im wondering if and how its possible to list only child pages under their respective parents? for example i have 3 pages; trees, sports, and houses. when i click trees, i want the sidebar to only show pines, oaks, etc. when i click oak, it all stays shown, just oak is highlighted with .active

i see where it can be restricted for certain pages, but can it be dynamic like so?

thanks!