Is there a way to open new child windows or switch between open child windows in the WinBox GUI?
What are the WinBox hotkeys and keyboard combinations that manage child windows?
Are there any keyboard shortcuts that I can use with the WinBox GUI?

Almost all of those are for the CLI.
There must be more hotkeys for the GUI than F6 (child window focus switch)?

Shift + F6 only reverses the window switching order.