Hibernate is not creating new child entities while updating existing parent entity

Question

I have a One to Many cardinality relationship like the following -

@Entity
@Table(name = "PARENT_TABLE")
public class Parent {

    @Id
    @GeneratedValue(generator = "uuid2")
    @GenericGenerator(name = "uuid2", strategy = "uuid2")
    @Column(name = "PARENT_ID", updatable = false, nullable = false, length = 16)
    @Type(type="uuid-binary")
    private UUID id;

    private String parentName;

    @OneToMany(fetch=FetchType.LAZY, mappedBy = "parent", cascade = CascadeType.ALL, orphanRemoval = true)
    @JoinColumn(name="PARENT_ID")
    private List<Child> children;

}

@Entity
@Table(name = "CHILD_TABLE")
public class Child {

    @Id
    @GeneratedValue(generator = "uuid2")
    @GenericGenerator(name = "uuid2", strategy = "uuid2")
    @Column(name = "CHILD_ID", updatable = false, nullable = false, length = 16)
    @Type(type="uuid-binary")
    private UUID id;

    private String childName;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "PARENT_ID", referencedColumnName = "PARENT_ID")
    private Parent parent;

}

Now, I already have a parent with 2 children, I have an update call for parent which is updating some information in parent and adding one child in parent.

List<Child> existingChild = getFromDbByParent(parent);
existingChild.add(new Child());
hibernateTemplate.update(parent);
//hibernateTemplate.saveOrUpdate(parent);

The above code is failing, because for the 3rd child, hibernate is not creating ID while saving to DB. So, I am getting error

2019-12-19 13:02:04.589 DEBUG SqlExceptionHelper             io-12347-exec-1 could not execute batch [update CHILD_TABLE set CHILD_ID =null where PARENT_ID=?]

java.sql.BatchUpdateException: ORA-01407: cannot update ("DB"."CHILD_TABLE"."CHILD_ID") to NULL

Creation of parent along with new children is working fine, even with JPA Cascade annotations. What can be done here?

Note: It's a repo layer migration to hibernate project, so I am trying to avoid changes in logic which is Why to save new child in existing parent. I know, it's better to call save on child first.