Could you please elaborate more on your scenario? Is this actually the desired scenario :
How does the ToggleButton know when to disappear for items that have no children?
ToggleButton placed only on the RadTreeViewItems that have children ? If it is something similar you can have different HeaderTemplates for the items and use the RadTreeView's ItemTemplateSelector. You can read more about it in this help article. The button in the template may change a ViewModel's property bound to the Visibility of other control in the HeaderTemplate.
the Telerik team