childtoparentback.java

Observable.from(parents)
  .flatMap(Observable::from)
  .map(parent -> parent.children)
  .flatMap(Observable::from)
  .filter(c -> c.index % 2 == 0)
  .groupBy(child -> child.parent)
  .flatMap(Observable::toList)
  .map(children -> {
      Listing child = children.get(0); // i know there is at least one
      child.parent.children = children;
      return child.parent;
  })
  .subscribe(...)