From ReactOS

typedef struct _MDI
{
  ULONG cKids;          // 000
  HWND hwndMaxedChild;  // 004
  HWND hwndActiveChild; // 008
  HMENU hmenuWindow;    // 00c
  ULONG idFirstChild;   // 010
  WORD wScroll;         // 014
  PVOID pTitle;         // 018
  INT iChildTileLevel;  // 01c
} MDI, *PMDI;

// Window Extra data container.
typedef struct _MDIWND
{
  WND wnd;          // 000
  DWORD dwReserved; // 098
  PMDI pmdi;        // 09c
} MDIWND, *PMDIWND;

References

• Windows Symbol files, userkdx.dll, !dso