/*  DLL source file   */

#include <vcl.h>
#include <windows.h>

#include "DllMDIChild.h"
#include "MDIChild.h"

#pragma hdrstop

USEFORM("MDIChild.cpp", ChildForm);
USE("DllMDIChild.h", File);
//---------------------------------------------------------------------------
TApplication* DllApp = 0;

int WINAPI DllEntryPoint(HINSTANCE hinst, unsigned long reason, void*)
{
        return 1;
}
//---------------------------------------------------------------------------
extern "C" DLL_EXP void __stdcall ShowMDIChildForm(TApplication* mainApp)
{
   if (!DllApp) {
        DllApp = Application;
        Application = mainApp;
   }

   TChildForm* child = new TChildForm(Application->MainForm);
   child->Show();
}
//---------------------------------------------------------------------------
extern "C" DLL_EXP void __stdcall ResetDllApplication()
{
   if (DllApp) {
        Application = DllApp;
   }
}
//---------------------------------------------------------------------------

/*  DLL header file   */

#ifdef __DLL__
#define DLL_EXP __declspec(dllexport)
#else
#define DLL_EXP __declspec(dllimport)
#endif

extern "C" DLL_EXP void __stdcall ShowMDIChildForm(TApplication* mainApp);
extern "C" DLL_EXP void __stdcall ResetDllApplication();


/*   MDI parent  */

//---------------------------------------------------------------------------
#include <vcl.h>
#pragma hdrstop

#include "MDIParent.h"

// Child Dll header

#include "DllMDIChild.h"

//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TMainForm *MainForm;
//---------------------------------------------------------------------------
__fastcall TMainForm::TMainForm(TComponent* Owner)
        : TForm(Owner)
{
}
//---------------------------------------------------------------------------
void __fastcall TMainForm::FormCloseQuery(TObject *Sender, bool &CanClose)
{
   ResetDllApplication();
}
//---------------------------------------------------------------------------

void __fastcall TMainForm::X1Click(TObject *Sender)
{
   Close();       
}
//---------------------------------------------------------------------------

void __fastcall TMainForm::FormClose(TObject *Sender, TCloseAction &Action)
{
   Action = caFree;       
}
//---------------------------------------------------------------------------

void __fastcall TMainForm::N2Click(TObject *Sender)
{
   ShowMDIChildForm(Application);       
}
//---------------------------------------------------------------------------

void __fastcall TMainForm::B1Click(TObject *Sender)
{
   Previous();
}
//---------------------------------------------------------------------------

void __fastcall TMainForm::F1Click(TObject *Sender)
{
   Next();       
}
//---------------------------------------------------------------------------

물론 부모 프로젝트엔 자식폼이 있는 DLL 파일의 *.lib 파일이 있어야 됩니다.

왜 그런지는 자세히 보시고 생각해보세요.