MDI? In WPF?! You must be joking! MDE is discouraged, so WPF does not support it.
I wonder who may want MDI with its completely broken reputation, a style which was bad from the very beginning and nobody liked it? You can use tabbed interfaces, docking UI (did you ever see AvalonDock:
http://avalondock.codeplex.com/[
^] which closely matches UI features of Visual Studio?)
(What's funny, I found a work used to mimic MDI with WPF. Forced regress in action:
http://wpfmdi.codeplex.com/[
^])
—SA