Taxonomies aren’t children of one another. Taxonomy terms can be hierarchical to where they can have parent/child relationships (like categories.)
When registering a taxonomy, you just make sure the hierarchical toggle is on if that’s what you want. If it’s not on, the terms cannot have parent/child relationships and be nested. If hierarchical is on, they can be and when you go to create terms in that taxonomy, you should see the ability to set an existing term within that taxonomy as the parent.
