There is no concept of child window in
System.Windows.Forms and not in WPF, except classical MDI. You're not using it (and this is a perfect decision: MDI is one of the worst UI inventions by Microsoft, it is highly discouraged by anyone including Microsoft and never used in more or less acceptable quality applications).
You're trying to use
TabControl based UI, which is very good. In such design, there are no other windows except a main form with
TabControl (well, they can be, but not related to your tabbed interface). An instance of
TabControl has tab pages which contain children control (they can be windowed controls in case of
System.Windows.Forms, but they are not windows or forms from the stand point of
System.Windows.Forms or WPF; in WPF children can be instances of
UIElement.
This is one of the articles showing how to make such interface:
Tabbed MDI in WPF[
^]. Griff provided a link to the solution for Forms.
(Sorry, you did not tag your library, so I've chosen WPF; you can search CodeProject or Google for more samples.)
—SA
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