Child Form에 CreateParam을 Override 시켜 Style를 변경 시켜주면 됩니다.

unit Unit2;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs;

type
  TForm2 = class(TForm)
  private
    { Private declarations }
  public
    { Public declarations }
    procedure CreateParams(VAR Params: TCreateParams); override;
  end;

var
  Form2: TForm2;

implementation

{$R *.DFM}

{ TForm2 }

procedure TForm2.CreateParams(var Params: TCreateParams);
begin
  Inherited Createparams(Params);
  WITH Params DO
    Style := (Style OR WS_CHILD OR WS_BORDER) AND NOT WS_DLGFRAME;
end;

end.