Well, don't do it. What are you trying to do would be the abuse of MDI design. Moreover, MDI itself is the abuse of reasonable UI design. Who seriously use MDI?
Sorry for not answering your question; but listen to a good friendly advice: don't use this feature strongly discouraged by virtually everyone, first of all by Microsoft. Stop torturing yourself by unnatural problems and scaring off your users by ugly usability and presentation.
See:
http://en.wikipedia.org/wiki/Multiple_document_interface#Disadvantages[
^],
Question on using MDI windows in WPF[
^],
MDIContainer giving error[
^].
My past answers references above describe some reasonable alternative. See also the related discussions.
—SA