@Entity
public class Parent implements Serializable{
    @Id @GeneratedValue
    public Long id;
    @OneToMany(cascade=CascadeType.ALL, orphanRemoval=true)
    public List<Children> childs = new ArrayList<Children>();
}

@Entity
public class Children implements Serializable {
    @Id @GeneratedValue
    public Long id;
}

public static void main(final String[] args) {
        EntityManager em = createEntityManager();

        Parent p = new Parent();
        Children c1 = new Children();
        Children c2 = new Children();
        p.childs.add(c1);
        p.childs.add(c2);
        
        EntityTransaction transaction = em.getTransaction();
        transaction.begin();
        em.persist(p);
        transaction.commit();
        em.close();
        
        // simulate a new Client Request which always get a new EntityManager
        
        em = createEntityManager();
        Parent pNeu = new Parent();
        Children c1Neu = new Children();
        pNeu.id = p.id;
        c1Neu.id= c1.id;
        pNeu.childs.add(c1Neu);
        transaction = em.getTransaction();
        transaction.begin();
        em.merge(p);
        transaction.commit();
        em.close();
        
        // simulate a new Client Request which always get a new EntityManager
        
        em = createEntityManager();
        pNeu = new Parent();
        pNeu.id = p.id;
        c1Neu = new Children();
        c1Neu.id= c1.id;
        Children c2Neu = new Children();
        p.childs.add(c1Neu);
        p.childs.add(c2Neu);
        transaction = em.getTransaction();
        transaction.begin();
        em.merge(p);
        transaction.commit();
        em.close();
    }