You cannot mix parenting controls by a MDI form with parenting MDI children in any simple way.
Why suffering from the broken design of MDI, which is highly discouraged by nearly everyone, first of all — by Microsoft and extremely inconvenient for users? Did you ever see more or less new software which is at least professional, using MDI? You won't find any. So why torturing yourself and chasing off your users?
There are easy and neat alternatives which you really want to use.
Please see:
http://en.wikipedia.org/wiki/Multiple_document_interface#Disadvantages[
^],
Question on using MDI windows in WPF[
^],
MDIContainer giving error[
^],
How to Create MDI Parent Window in WPF?[
^].
The simplest decent alternative is using tabbed interface based on
TabControl
. What you have now in the form of MDI children will be the tab pages.
—SA