MDI form stays ontop but parent is still enabled

I have a MDI form with a child MDI form being displayed by a SDI application.  I have the MDI form ontop of all other forms working fine, but the SDI is still active [enabled] and I don't want the user to be able to close it without closing the MDI form first.

I tried disabling the SDI form before showing the MDI form with the vbModal parameter, then enabling it after the .Show call, but vbModal doesn't seem to work with MDI forms - I've read that too.

How can I get the SDI form to become disabled when the MDI form is active, then get enabled after the user closes the MDI form?

Thanks.