SendMessage(*hChild, WM_COMMAND, 0, (LPARAM)((LPTSTR)"Соединение установлено"));

case WM_COMMAND:
        stttr = ((LPTSTR)(pcds->lpData));
        InvalidateRect(hChild, NULL, TRUE);
        break;
    case WM_PAINT:
    {
        HDC hdc = BeginPaint(hChild, &ps);
        TextOut(hdc, 10, 10, stttr, 50);//вывод текста

        EndPaint(hChild, &ps);
    }
        break;

COPYDATASTRUCT* pcds = (COPYDATASTRUCT*)lParam;
LPTSTR stttr = "Ожидание";