I am in the same situation where I want to display only child pages of the current page in the Site PanelBar.
I had set the StartFromCurrentNode property to true and ShowStartingNode property to false. In the Site Panel Bar.
However the menu in the template has the StartingNodeUrl set to the Home page which is a grouping node.
In this scenario I want to show only subpage1 and subpage2 in the Site Panel Bar when I am positioned on page3.
At the same time I want the breadcrums displaying the Home page node:
Home>page3>subpage1. At this point the breadcrums work fine with the Site Menu but the Site Panel Bar does not display anything. You have said that the StartingNodeUrl and StartFromCurrentNode are mutual exclusive which I assume this is the reason why the SitePanelBar is not displaying. Given this situation how can I display the SitePanelBar?