How Do I: Have a MDIChild open another MDIChild in the MDIParent?

Hello:
I have a MDIParent that opens a MDIChild and it works fine. Now I need the MDIChild open another form and open it into the MDIParent. I tried the following code and I get an error. When I comment the error line, it does show the form but its not in the MDIParent. What am I missing?
 
FormRecord FormToShow = new FormRecord(listBoxField_ID, listBoxField_Value);
//FormToShow.MdiParent = this; // <<< ERROR
FormToShow.Show();