type
  IMyAncestor = interface
    procedure P1;
  end;

  IMyChild = interface(IMyAncestor)
    procedure P2;
  end;

  TMyChild = class(TObject, IMyChild)
    procedure P1;
    procedure P2;
  end;

implementation

{ TMyChild }

procedure TMyChild.P1;
begin
  ShowMessage('P1');
end;

procedure TMyChild.P2;
begin
  ShowMessage('P2');
end;