Type MyMDIClass = class of TMdiBaseForm;
Var   MyMDIForms: TDictionary<MyMDIClass, TMdiBaseForm>

function TMainForm.CheckMDIChild(Sender: TForm): Boolean;
var
  i: Integer;
begin
  Result := False;
  for i := 0 to (MDIChildCount - 1) do
    if MDIChildren[i] = Sender then
    begin
      Result := True;
      exit;
    end
    else
      Result := False;
end;

function TMainForm.CheckMDIChild(Sender: TForm): Boolean;
begin
  Result := MyFormsRegistry.Contains(Sender as TMdiBaseForm);
end;

Type TMdiBaseForm = class
...
private
    FRegistry: TList<TMdiBaseForm>; 
   // может быть один синглтон на всю программу, или несколько (разные открытые проекты-странички-файлы)
....
public
  procedure BeforeDestruction; Override;
  property MDIRegistry: TList<TMdiBaseForm> write RegisterInNewRegistry;
....

procedure TMdiBaseForm.BeforeDestruction; 
begin
   MDIRegistry := nil;
   inherited;
end;

procedure TMdiBaseForm.RegisterInNewRegistry(const NewReg: TList<TMdiBaseForm>); 
begin
   if NewReg = FRegistry then exit;
   if nil <> FRegistry then FRegistry.Remove(Self);
   if nil <> NewReg then NewReg.Add(Self);   
   FRegistry := NewReg;
end;

.....

  LMyNewForm := TMyMDIxxxxForm.Create( ...... );
  LMyNewForm.XXX := yyy;
  LMyNewForm.AAA := bbb;
  LMyNewForm.MDIRegistry := MyProjectWindows;
  LMyNewForm.Show;

procedure TForm2.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action := caFree;

  Form2 := nil;
end;[/src[

Ну [SRC pascal]Sender:= nil;

procedure TMdiBaseForm.RegisterInNewRegistry(const NewReg: TList<TMdiBaseForm>); 
begin
   if NewReg = FRegistry then exit;
//   if nil <> FRegistry then FRegistry.Remove(Self);
   if nil <> FRegistry then begin
     FRegistry.Remove(Self);
     FRegistry := nil;
   end;
   if nil <> NewReg then NewReg.Add(Self);   
   FRegistry := NewReg;
end;