I am not quite sure that 3.x will allow this, setting it from the menu. 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. you can retrieve the child pages of how do i
page using the pages API
(either fluent or native).
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