You probably fixed this by now, but for future visitors - you can do this with no code - on any page.
The ‘parent 0’ trick only works for main categories - if you go to a subcat, it breaks (because these have parents).
You can get ONLY the NEXT CHILD Categories by putting {echo:get_queried_object_id}
In both PARENT and CHILD OF - in the query.
In a category archive, this returns just same-level categories, enabling you to ‘walk down’ the category tree and show ‘next branches’ to go down - with a single template.
So on you store it would show DIY, FASHION, OUTDOOR
Go to OUTDOOR - and the same archive template shows climbing, running, hiking
If you had Hiking->boots, trousers, gear
Then on Hiking page it would show those… ad infinitum.