code: public List<T> getChildren(T payload) {
      List<T> children = new ArrayList<>();
      if (!mIndex.containsKey(payload)) {
        return children;
      }
      DirectedAcyclicGraphNode<T> node = mIndex.get(payload);
      for (DirectedAcyclicGraphNode<T> child : node.getChildren()) {
        children.add(child.getPayload());
      }
      return children;
    }
docs: Gets the payloads for the children of the given node.
  
  @param payload the payload of the parent node
  @return the children's payloads, an empty list if the given payload doesn't exist in the DAG