I've done some testing and I can confirm that if you follow the 4.1

A

public class Parent{
@OneToMany(cascade={ CascadeType.ALL },orphanRemoval=true,fetch=FetchType.LAZY)
@JoinColumn(name="FK_ParentId",nullable=false)
@LazyCollection(LazyCollectionOption.EXTRA)
@OrderColumn(name="idx")
public List<Child> getChildren()

[...]
public class Child{
//no idx attribute
@ManyToOne(fetch=FetchType.LAZY,optional=false)
@JoinColumn(name="FK_ParentId",nullable=false, insertable=false,updatable=false)
public Parent getParent() {

B

public class Parent{
@OneToMany(cascade={ CascadeType.ALL },orphanRemoval=true,mappedBy="parent",fetch=FetchType.LAZY)
@LazyCollection(LazyCollectionOption.EXTRA)
@OrderColumn(name="idx")
public List<Child> getChildren()

[...]
public class Child{

@Column
private int idx;

@ManyToOne(fetch=FetchType.LAZY,optional=false)
@JoinColumn(name="FK_ParentId",nullable=false)
public Parent getParent() {

documentation(http://docs.jboss.org/hibernate/stable/annotations/reference/en/html_single/#entity-hibspec-collection-extratype-indexbidir), and
A. remove the mappedBy, ExtraLazy access seems to be out of the window. Every collection.get leads to full initialization of the collection.

B. with the other option in the documentation where mappedBy is used and an extra idx attributes is added to the child, I'm seeing the idx being 0 every time and getting duplicate inserts (Hibernate 4.1.7).

In both cases I'm doing
child.setParent(parent);
parent.getChildren().add(child);

parent = dao.save(parent);

dao.flush();

case A saves correctly, but ExtraLazy element fetching doesn't work.
case B, I'm getting first child insert on save and a second one when I call dao.flush().
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