I made a form using the Qt Designer which has some dockwidgets, these dockwidgets have some children widgets. How I can access the dockwidget and these child widgets in my mainwindow.cpp?
I highly recommend reading the docs for these kinds of things, but to give you a little head start, QDockWidget inherits from QWidget, which inherits from QObject:
https://doc.qt.io/qt-4.8/qobject.html#children
widget->children() would simply tell you the children of this widget. This would be needed if you didn't already know the names of the objects to be accessed directly, or had no reference to them.
Update
When you create objects in Qt Designer, and you run the setupUi(this) that is generated for you, inside of your MainWindow, you will then have access to all of the widgets you had set up as members. You can access them directly as they were named in Qt Designer. Please check out one of the numerous tutorials on getting started with Qt. Here is one that shows you how to make use of your ui file, and access the members from it: http://sector.ynet.sk/qt4-tutorial/my-first-qt-gui-application.html