I just discovered, through experimentation, that if I hold the ALT key and close a tab, all tabs except that tab get closed.
Next I discovered that if I hold CTRL and close a tab with children, both the tab I'm closing and all its children are closed.
That's awesome!
But how would a user ever know about this?
I propose you include this information in the tab preview that appears to the right of a tab when the pointer is hovering over a tab. Enable it by default and add a setting so users can turns it off if it bothers them.
This would greatly help users to discover Orion's already awesome and nifty tab close behavior with keyboard modifiers.