class CChildFrame : public CMDIChildWnd
{
   // This makes the MDI child maximized.
   virtual void ActivateFrame(int nCmdShow)
   {
       // if another window is open, use default
       if(GetMDIFrame()->MDIGetActive())
           CMDIChildWnd::ActivateFrame(nCmdShow);
       else // else open maximized.
           CMDIChildWnd::ActivateFrame(SW_SHOWMAXIMIZED);
   }
  
};

자료 참고 : MFC FAQ ver 5.6
         - Stephen Bade, bade@convergent-design.com