The documentation says that the 'get child' callback is optional:

> To implement, each plugin can have a 'get child' and 'get children' callback. Both of these should be implemented for performance reasons, since it is best to avoid getting all children if necessary, but if 'get child' is not implemented, it will fall back to 'get children' if it has to.

But the code will produce a warning if it's not defined, and there is no fallback:

    // If we are looking for a child, and have the parent, ask the parent for the child.
    if (!empty($child) && !empty($plugins[$module][$type][$parent]) && function_exists($plugins[$module][$type][$parent]['get child'])) {
      $plugins[$module][$type][$id] = $plugins[$module][$type][$parent]['get child']($plugins[$module][$type][$parent], $parent, $child);
    }