GVis Among the available hotkeys, which by default have no key assigned, there is the hotkey for selecting all descendant bones:
You can set any key here and enable this hotkey by pressing the Reload
button in the Spine Settings window or restarting Spine.
For your information, the hotkey Select - Child Bones
selects only the direct child of the selected bone(s), so if you want to select even the child bones that the direct child bone has, as you have shown in the screenshot, use the Select - Descendant Bones
hotkey.