Inside drop-down menu child menus are not working
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.