A child window is a window that is contained within a larger window (which could
be a child of yet another window). The sub-windows of an MDI application (see
%fr and %aw) are children, as are the separate sheets of a property sheet (see
%ps and %sh). However, most controls, such as buttons, are considered by
Windows to be children of the window that contains them. This is why controls
can be referred to by their window handle (see %lc).