Thank you for contacting Telerik Support.
If a parent page is set as non-navigable it will not be part of the SiteMap - this means that when you try to bind the menu to a non existing node you would get an error.
Instead of binding your widget to a SiteMapDataSource control you can bind it to a collection of pages which later you can use to retrieve the child pages a page using the pages API (either fluent or native) and hide them.
The ability to automatically hide pages for users who are not allowed to view them is planned for Q2.
I hope this information helps. I will be happy to assist you if you have any further questions.
the Telerik team