The RadTreeView basically represents hierarchical data but in your case you want something different - for example property of an item to behave like its child. When binding the RadTreeView, every hierarchy level needs objects that inherit from one and the same type. That is why in your case you need to make some of the properties and the child items to inherit from common type. I prepared a possible approach for you and you can examine it in the attached solution.
On a side note, have you considered using a RadTreeListView?
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