Like the frame window procedure, an MDI child window procedure uses a special function for processing messages by default. All messages the child window procedure does not handle must be passed to the DefMDIChildProc function rather than to the DefWindowProc function. In addition, some window-management messages (such as WM_SIZE, WM_MOVE, and WM_GETMINMAXINFO) must be passed to DefMDIChildProc even if the application handles the message, in order for MDI to function correctly. For a complete list of messages the application must pass to DefMDIChildProc, see the Microsoft Windows Programmer's Reference, Volume 2.
The child window procedure in Multipad is named MPChildWndProc.