int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    QWidget *Window = new QWidget();
    Window->setWindowTitle("Fenêtre parent");
     Window->setGeometry(10,10,600,400);
   Window->show();


    QWidget *childWindow = new QWidget(Window); 
    childWindow->setWindowTitle("Fenêtre enfant");
    childWindow->setGeometry(20,20,300,200);
    childWindow->show();


    return QCoreApplication::exec();

Merci de votre aide.