MDI Child form automatically in a maximized state when another child form is in a maximized state

You resize window gets WM_WINDOWPOSCHANGING and WM_WINDOWPOSCHANGED commands pick them off and resize the client

In C/C++ you handle them in the window handler of the window that resizes like this so it will look something similar

// These next two messages are better to use rather than WM_MOVE/WM_SIZE.
// Remember WM_MOVE/WM_SIZE are from 16bit windows. In 32bit windows the window
// manager only sends these two messages and the DefWindowProc() handler actually
// accepts them and converts them to WM_MOVE/WM_SIZE.
// 
// We accept this so we can scale our controls to the client size.
     case WM_WINDOWPOSCHANGING:
     case WM_WINDOWPOSCHANGED:
     {
	RECT rc;
        HDWP hDWP;
         
        // Anywhere you see ??? you need to insert your value	

        // Create a deferred window handle.
        if(hDWP = BeginDeferWindowPos(???)){ // Deferring ??? of windows to move
           GetClientRect(hWnd, &rc);
 
           // Defer each window move/size until end and do them all at once.
           hDWP = DeferWindowPos(hDWP, ??? Handle to your window, NULL,
              ???x, ???y, ???cx, ???cy, SWP_NOZORDER | SWP_NOREDRAW);
 
           // RINSE AND REPEAT LINE ABOVE FOR EACH WINDOW YOU DEFERRED
           /* TIME FOR YOU TO ADD CODE */
 
           // Resize all windows under the deferred window handled together.
           EndDeferWindowPos(hDWP);
 
           // We told DeferWindowPos not to redraw the controls so we can redraw
           // them here all at once.
           RedrawWindow(hWnd, NULL, NULL, RDW_INVALIDATE | RDW_ALLCHILDREN |
              RDW_ERASE | RDW_NOFRAME | RDW_UPDATENOW);
        }		
        
     }
     return 0;