Inside drop-down menu child menus are not working I am using bootstrap sub-theme and created a drop-down menu and set it's menu levels they are working correctly but when i extend to child menus it's  not working.
I have searched Expand menu links down in a tree and Child navigation sidebar or drop-down navigation displaying child items but not found any solution.

Any Help?
Bootstrap 3 and 4 have dropped support for nested (multilevel) menus 

One has to include the relevant code (that was excluded) in their own custom theme to make it work.

Check this stackoverflow answer for relevant code to add to your theme 

NOTE: the reason the support was dropped is that multilevel nesting is problematic in mobile or smaller screens so you have to implement solutions for small screens on your own.