간단한 MDI애플리케이션을 만들어보고 있습니다.
MainMenu가 있는 창에 2개의 MDIChild폼을 생성하는건데 말이죠,
Inspector에서 MainMenu창의 FormStyle을 fsNormal로 했을때는 제대로 컴파일은
되지만, 의도한바가 아니듯이, MainMenu Form내에 ChildForm이 생성되지 아니하고,
외부에 창이 생성됩니다. 총 3개의 FormStyle을 fsMDIChild로 지정해주고 컴파일을
시도하면, 다음과 같은 에러메세지를 내면서, CPU창이 뜹니다.
4번이나 재차 프로젝트를 시도해봤지만, 절대루 안되네요. @,@;;;;...!
파일도 첨부했습니다. 도움이 될려나 모르겠지만.. 이렇게 무식하게 올려봅니다.

┌──────────────────┐     MDIChild_2 : Unit1.cpp
│File    Window                      │     MDIChild_3 : Unit3.cpp
├┬─────────────────┤     MainMenu   : Unit3.cpp
││              ┌───────┐  │
│├─MDIChild_1  │ MDIChild_1   ├┐│     ----------------------
││              └─┬─────┘││           파일첨부
││                  │ MDIChild_2 ││     ----------------------
│└─MDIChild_2      └──────┘│     Borland C++ Builder 3
├─────┐            ( Ｏ )      │
│Minimized │   FormStyle=fsMDIChild │  
└─────┴────────────┘

┌────────────────────────────────────┐
│Error                                                               [x] │
├────────────────────────────────────┤
│[Ｘ] Project Project1.exe raised exception class EinvalidOperation with │
│     message 'Cannot create form. No MDI forms are currently active'.   │
│     Process stopped. Use Step or Run to continue.                      │
│                        ┌───────┐                              │
│                        │     ＯＫ     │                              │
│                        └───────┘                              │
└────────────────────────────────────┘

//-------------------------------------------------------------------------
//unit1.h
//-------------------------------------------------------------------------
//---------------------------------------------------------------------------
#ifndef Unit1H
#define Unit1H
//---------------------------------------------------------------------------
#include <Classes.hpp>
#include <Controls.hpp>
#include <StdCtrls.hpp>
#include <Forms.hpp>
#include <Menus.hpp>
//---------------------------------------------------------------------------
class TForm1 : public TForm
{
__published: // IDE-managed Components
    TMainMenu *MainMenu1;
    TMenuItem *File1;
    TMenuItem *FirstChild1;
    TMenuItem *SecondChild1;
    TMenuItem *N1;
    TMenuItem *Exit1;
    TMenuItem *Window1;
    TMenuItem *Tile1;
    TMenuItem *Cascade1;
    TMenuItem *ArrangeIcons1;
    TMenuItem *N2;
    TMenuItem *ActiveChild1;
    TMemo *Memo1;
    void __fastcall FirstChild1Click(TObject *Sender);
    void __fastcall SecondChild1Click(TObject *Sender);
    void __fastcall Tile1Click(TObject *Sender);
    void __fastcall Cascade1Click(TObject *Sender);
    void __fastcall ArrangeIcons1Click(TObject *Sender);
    void __fastcall ActiveChild1Click(TObject *Sender);
    void __fastcall Exit1Click(TObject *Sender);
private: // User declarations
public:  // User declarations
    __fastcall TForm1(TComponent* Owner);
};
//---------------------------------------------------------------------------
extern PACKAGE TForm1 *Form1;
//---------------------------------------------------------------------------
#endif

//---------------------------------------------------------------------------
//unit.cpp
//---------------------------------------------------------------------------
//---------------------------------------------------------------------------
#include <vcl.h>
#pragma hdrstop
#include "Unit1.h"
#include "Unit2.h"
#include "Unit3.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
    : TForm(Owner)
{
}
//---------------------------------------------------------------------------

void __fastcall TForm1::FirstChild1Click(TObject *Sender)
{
    Application->CreateForm(__classid(TForm2), &Form2);
    Form2->WindowState=wsNormal;
    Form2->Show();
}
//---------------------------------------------------------------------------

void __fastcall TForm1::SecondChild1Click(TObject *Sender)
{
    Application->CreateForm(__classid(TForm2), &Form2);
    Form2->WindowState=wsNormal;
    Form2->Show();
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Tile1Click(TObject *Sender)
{
    Tile();   
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Cascade1Click(TObject *Sender)
{
    Cascade();   
}
//---------------------------------------------------------------------------
void __fastcall TForm1::ArrangeIcons1Click(TObject *Sender)
{
    int i;
    for(i=MDIChildCount-1; i>=0; i--)
        MDIChildren[i]->WindowState=wsMinimized;
}
//---------------------------------------------------------------------------
void __fastcall TForm1::ActiveChild1Click(TObject *Sender)
{
    ShowMessage(ActiveMDIChild->Caption);
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Exit1Click(TObject *Sender)
{
    Close();
}
//---------------------------------------------------------------------------
//Unit2.h
//---------------------------------------------------------------------------
#ifndef Unit2H
#define Unit2H
//---------------------------------------------------------------------------
#include <Classes.hpp>
#include <Controls.hpp>
#include <StdCtrls.hpp>
#include <Forms.hpp>
//---------------------------------------------------------------------------
class TForm2 : public TForm
{
__published: // IDE-managed Components
    TLabel *FirstChildForm;
private: // User declarations
public:  // User declarations
    __fastcall TForm2(TComponent* Owner);
};
//---------------------------------------------------------------------------
extern PACKAGE TForm2 *Form2;
//---------------------------------------------------------------------------
#endif
//---------------------------------------------------------------------------
//Unit2.cpp
//---------------------------------------------------------------------------
#include <vcl.h>
#pragma hdrstop

#include "Unit2.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm2 *Form2;
//---------------------------------------------------------------------------
__fastcall TForm2::TForm2(TComponent* Owner)
    : TForm(Owner)
{
}
//---------------------------------------------------------------------------
//Unit3.h
//---------------------------------------------------------------------------
//---------------------------------------------------------------------------
#ifndef Unit3H
#define Unit3H
//---------------------------------------------------------------------------
#include <Classes.hpp>
#include <Controls.hpp>
#include <StdCtrls.hpp>
#include <Forms.hpp>
//---------------------------------------------------------------------------
class TForm3 : public TForm
{
__published: // IDE-managed Components
    TLabel *SecondChildForm;
private: // User declarations
public:  // User declarations
    __fastcall TForm3(TComponent* Owner);
};
//---------------------------------------------------------------------------
extern PACKAGE TForm3 *Form3;
//---------------------------------------------------------------------------
#endif
//---------------------------------------------------------------------------
//Unit3.cpp
//---------------------------------------------------------------------------
#include <vcl.h>
#pragma hdrstop

#include "Unit3.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm3 *Form3;
//---------------------------------------------------------------------------
__fastcall TForm3::TForm3(TComponent* Owner)
    : TForm(Owner)
{
}
//---------------------------------------------------------------------------

여기까지 보신분 계시나여?  ^^;  흘~ --;  이 수밖에 엄네요~ -.-!