OK, warming up my brain :signOops: I remembered that I got through this problem some (longer) time ago.
The fact is: You can't add a panel to a panel directly
The trick is: Change the parent of the sub-panels to the main panel at the beginning of the application using the FormLib
Now the sub panels belong to the parent panels and e.g. are un-/visible if you hide/show the main panel (see attachment).