For performance reasons, the topics breadcrumb are not generated right after you add parent/children topics.
However, you can manually trigger breadcrumb generation by:
1. logging into your community as an admin
2. open this link below in a new tab: https://yourcommunityurl.tribe.so/api/v1/actions/topics65
Please make sure you replace yourcommunityurl.tribe.so41 with your community's url.
It will give you a "success" message in the tab, and at that moment all the breadcrumbs and the contents inside the parent-child topic pages get updated.