The 'Number Children' shortcut (Ctrl+Shift+X) doesn't work

Ctrl+Shift+X shortcut doesn’t work

Works for me. You probably have something else assigned to the hotkey. Your OS, other apps, your browser, your browser extensions, they all map hotkeys and if it’s already set somewhere else, it won’t work. That happens to all apps on all computers, just how it is. If you want to fix the conflict you can download a detector like shortcutdetective, or if you have a Pro subscription you can change the shortcut to something else.

Thanks @BigChungus. I had something else clashing with it here - chrome://extensions/shortcuts.