Sub Categories in drop down menu

You can use this function:

the_widget( 'WP_Widget_Categories', 'dropdown=1&hierarchical=1' );

This function give you a select box and you will get all the parent categories and sub-categories. After applying this code inspect category select box, then you can manipulate your HTML code. And show your category as you wish.

If you don’t want any select box, then you just remove "downdown=1&". This will give you a whole list of all categories and sub-categories. After applying this code inspect category select box, then you can manipulate your HTML code. And show your category as you wish.

Hope this will help!