Use WS_VSCROLL and WS_HSCROLL styles to create an MDI client window that allows respectively vertical or horizontal bars in the MDI child windows.
hwndClient = CreateWindow (TEXT ("MDICLIENT"), NULL,
WS_CHILD | WS_CLIPCHILDREN | WS_VISIBLE | WS_VSCROLL | WS_HSCROLL,
0, 0, 0, 0, hWnd, (HMENU) 1, hInst,
(PSTR)&clientcreate) ;