Hello Michael,
Unfotunately this is a limitation in the current Silverlight version - this scenario is supported in WPF, but not in Silverlight.
Why you just do not set the IsLoadOnDemandEnabled property to the RadTreeView? It will take care to set the "expander image" on the nodes that do not have children.
Best wishes,
Valentin.Stoychev
the Telerik team
Check out
Telerik Trainer, the state of the art learning tool for Telerik products.