The LoadOnDemand feature of RadTreeView applies only when the node is expanded. Have a look at our online LoadOnDemand example
for further reference. Populating a family with members when this family is checked is another story. First, you will have to register a handler for RadTreeViewItem's routed event CheckedEvent:
The event handler(treeViewItem_Checked) will get invoked every time an item is checked, no matter if it is of type Family or Member. Therefore, you will have check for the type of the item and if it is of type Family, then load the members of that family:
sender, Telerik.Windows.RadRoutedEventArgs e)
RadTreeViewItem item = e.OriginalSource
Family family = item.DataContext
I have changed the business model a little bit so it fits the scenario better. Basically, that is the idea. Have a look at the attached sample project and let me know if you have additional questions or something is unclear.
the Telerik team