Hi Christian,

In order to create are child menu in joomla you have to create new menu at menus manager then create another menu items inside you new menu, after that you have to create new modules at extensions-> Modules then you have to name that module and also position it follow you theme.

for more understanding you can check this document:- https://docs.joomla.org/J3.x:Creating_a_submenu

Hope this Help and have a nice day.