You can use Views to achieve the structure of sub-terms under parent Terms. Views are a User interface for sql searches so you are setting the definition for the query that will be made of the content database.

Steps to create sub-terms under parent Terms using views:--

• Create a view to display a taxonomy term with a page with a path configured.
• Add Taxonomy term: Term ID in CONTEXTUAL FILTERS.
• Add Taxonomy term: Parent term in RELATIONSHIPS.
• Add Taxonomy term: Parent term in Filter criteria section. select great than 0.
• add Taxonomy term: Rendered Taxonomy term to your Fields, select relationship to parent and check the Exclude from display.
• In setting, under FORMAT section. In grouping field 1, select (Parent) Taxonomy term: Rendered Taxonomy term.
• Under Sort criteria, add Taxonomy term: Name, Sort ascending or descending and select parent under relationship.

  

Your view preview should look like

Now, Check your Page display will output as a tree

Style this page as your requirement.