import sys
from PySide6.QtWidgets import QApplication, QMainWindow, QPushButton, QDialog, QVBoxLayout

class ParentWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.setWindowTitle("Родительское окно")

        button = QPushButton("Открыть дочернее окно", self)
        button.clicked.connect(self.openChildWindow)

    def openChildWindow(self):
        child_window = ChildWindow(self)
        child_window.show()

class ChildWindow(QDialog):
    def __init__(self, parent=None):
        super().__init__(parent)

        self.setWindowTitle("Дочернее окно")

        layout = QVBoxLayout()
        layout.addWidget(QPushButton("Пример кнопки", self))
        self.setLayout(layout)

if __name__ == "__main__":
    app = QApplication(sys.argv)
    main_window = ParentWindow()
    main_window.show()
    sys.exit(app.exec_())