In this section:
%ch (Child Window) inserts a child window created by %`lw in a previous call to winio@. The format takes a single INTEGER argument that is the control variable of the child window.
Child windows are automatically restored on top of their parent and are constrained to lie within it. If they have a caption (as they will by default) they can be moved by the user. Child windows created in this way are fixed in size and location (relative to the parent window). Here is a simple example:
i=winio@('This is a child window%2nl%ch',control_var)