In order to send keystrokes to a child window or control when it's in the background, you must use this command. You must also use either the PostMesage or SendMessage send modes. These can be chosen explicitly with SetSendMode or implicitly with SendWinM or SendWinMF.
TargetChild looks for the specified child window inside the main window that is already targeted.
If no main window is targeted, TargetChild doesn't know where to look.
Therefore, before using TargetChild, you must use a command that targets a main window such as TargetWin, SendWin, SendWinMF, etc.
If you want to target a child of a child, call TargetChild repeatedly.