A topic is basically a way to organizing the content. Think of it as Tag in Facebook, or Hashtag in Instagram.
If I go to any topic page (whether a parent topic or a child topic) the page would contain all the posts that are tagged with that particular topic.
Now, if a content is tagged with a topic and it belongs to a "group", I can see the content in that topic page if the groups it belongs to is a "public group".
But if the content belongs to a "private" or "secret" group and it is tagged with a topic, I would not see the content in that topic page, unless I am a member of the group that this topics belongs to. This is the concept behind a secret or a private group. You should be a member of those groups to be able to see its content in any feed.
Also, if you create a group, and then add a topic as a "featured topics" to that group, it does not transfer the content from those topics into the group page. Groups are exclusive environments and inside each group feed, you only see the content which belongs to that group. Adding a highlighted topic to a group only helps you to filter the feed on the group to find the contents you are looking for in a shorter time.
If you are looking to give a way to users to filter the feed within a group, you can only use one level of topic. Here's how to feature topics on the left sidebar within a group: https://community.tribe.so/question/5d51a7f5ad5cf24c0cced69827
So, if you want to see a content in a group feed, you need to make sure that the content is posted in that specific group.