How to add Navigation Menu dropdown in Wordpress outside admin console?

How to add Navigation Menu dropdown in Wordpress outside admin console?

I am intending to use Wordpress Menu functionality i.e. creating a new
menu,re-arrange remove categories outside admin console.
Also any idea how to populate a dynamically menu being created without any
input form user end i.e. for a Parent Category if there are 10 child
categories we got to populate on our own a submenu with all 10 categories,
instead of asking the end user to add category one by one.
Let me know if you have any inputs from your end.Would be great to have
suggestions,inputs waited.