Claudiu, you're absolutely right in saying that it doesn't work

The first thing that comes to mind is that you want to override the child Window's OnMouseDown function and have it start a Drag and Drop operation. Then you can handle OnDragEnter in your main Window.

Better than overriding would be a helper class that has an attached property (WindowDockable?). Register a DependencyPropertyChangedHandler that is fired when the property is set on a window. In the handler add a listener for PreviewMouseDown on that Window. And the listener can start the Drag and Drop operation.

I love attached properties...they make adding functionality non-invasive (I.E. you don't have to subclass a class to add functionality to it).