To start off, if your application doesn't require MDI....refrain from using MDI capabilities. If you want the look, however, you can look into embedding forms within forms.
Embedding a form does two things, (1) In an MDI app, if you have controls on a parent form, the controls will be visible through the child form. Embedded forms automatically cover up anything in the "parent" form. (2) You will not have the problem of your child form defaulting to the state or the parent form.
Personally, if my application won't work SDI (Single Document Interface), I embed my forms. The only problem I have run into with embedded forms is my "child" form will not startup centerscreen.
Wrapping up, embedded forms, by default, behave better than a true MDI application. By the way, Microsoft actually frowns upon the use of MDI in real-world applications. I wish I could remember where I read that statement.