I solved above problem using Handled property of RadRoutedEventArgs.
In collapsed-event handler of child expander.
private void SelectionExpander_Collapsed(object sender, RoutedEventArgs e)
((Telerik.Windows.RadRoutedEventArgs)(e)).Handled = true;
After do that, collapsed-event is not propagated to event handler of parent expander.
If you have other solutions, please let me know that.