Had another go here.
D

public class Parent{

@OneToMany(cascade={ CascadeType.DELETE },orphanRemoval=false,fetch=FetchType.LAZY)
@JoinColumn(name="FK_ParentId")
@LazyCollection(LazyCollectionOption.EXTRA)
@OrderColumn(name="idx")
public List<Child> getChildren(){}
[..]

public class Child{

@ManyToOne(fetch=FetchType.LAZY)
public WallPost getPost() {}
[...]

parent.getChildren.add(element)

The entire children collection gets initialized. And because the parent now owns the relationship, you can't just save the child and be done with it.
The index counter does get inserted correctly here.

Is there any kind of workaround possible here?

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