Show only the Sub-Menues in the Side Bar

Hi

I couldn’t figure out how to do that:

I have a structue like this:

Parent 1
– Submenu 1a
— Sub-Submenu 1aa
— Sub-Submenu 1ab
— Sub-Submenu 1ac
— Sub-Submenu 1ad

– Submenu 1b
— Sub-Submenu 1ba
— Sub-Submenu 1bb
— Sub-Submenu 1bc
— Sub-Submenu 1bd

– Submenu 1c
— Sub-Submenu 1ca
— Sub-Submenu 1cb
— Sub-Submenu 1cc
— Sub-Submenu 1cd

Parent2
– Submenu 2a
— Sub-Submenu 2aa
— Sub-Submenu 2ab
— Sub-Submenu 2ac
— Sub-Submenu 2ad

– Submenu 2b
— Sub-Submenu 2ba
— Sub-Submenu 2bb
— Sub-Submenu 2bc
— Sub-Submenu 2bd

– Submenu 2c
— Sub-Submenu 2ca
— Sub-Submenu 2cb
— Sub-Submenu 2cc
— Sub-Submenu 2cd

Parent3

etc..

I have a Mainmenu with
Parent 1
– Submenu 1a
– Submenu 2a
– Submenu 3a

Now i just want that when i click Parent 1 in the MainMenu only the submenu appears on the leftsidebar and when i click “Submenu 1a” Only the specific Sub-Submenu appears.

How to do that?

Thank you
and Best regards
Christoph