<class name="Parent" table=".......
    <id name="parent_id" column="PARENT_ID_PARENT" type="long">
         <generator .................../>
    </id>
    <set name="children">
           <key column="CHILD_ID_PARENT"/>
           <one-to-many class="Child"/>
    </set>
</class>

<class name="Child" table=".......
   <id name="child_id" column="CHILD_ID_CHILD" type="long">
       <generator .....
   </id>
   ......
   ......    OTHER PROPERTIES OF CHILD
</class>

public class Parent {
   public long parent_id;
   public Set  children;
   ...

public class Child {
   public long child_id;
   ....
   ....

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping
    PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping>
    <class name="test.hibernate.Child" table="Child1">
        <id type="string" unsaved-value="null" >
            <column name="child_id_child" sql-type="varchar2(32)" not-null="true"/>
               <generator class="uuid.hex"/>
        </id>
        <property name="childName">
            <column name="childName" sql-type="char(30)" not-null="true"/>
        </property>
    </class>
</hibernate-mapping>   


<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping
    PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping>
    <class name="test.hibernate.Parent" table="Parent1">
        <id type="string" unsaved-value="null" >
            <column name="parent_id_parent" sql-type="varchar2(32)" not-null="true"/>
               <generator class="uuid.hex"/>
        </id>
        <property name="parentName" access="field">
            <column name="ParentName" sql-type="char(30)" not-null="true"/>
        </property>
        <set name="children">
              <key column="CHILD_ID_PARENT"/>
              <one-to-many class="test.hibernate.Child"/>
       </set>
    </class>
</hibernate-mapping>   

SQL> desc child1
Name                            Type
---------------------------- --------
CHILD_ID_CHILD           CHAR(32)
CHILD_ID_PARENT         CHAR(32)
CHILDNAME                   CHAR(30)

SQL> desc parent1
Name                             Type
--------------------------- ---------
PARENT_ID_PARENT     CHAR(32)
PARENTNAME               CHAR(30)

public class Parent {
    public String parentName;
    public Set children = new HashSet();

                public void addChild(Child child) {
             this.children.add(child);
                }
   public Set getChildren() {
      return children;
   }
   public void setChildren(Set children) {
      this.children = children;
   }
   public String getParentName() {
      return parentName;
   }
   public void setParentName(String parentName) {
      this.parentName = parentName;
   }
}


public class Child {
   private String childName;
   
   public String getChildName() {
      return childName;
   }
   public void setChildName(String childName) {
      this.childName = childName;
   }
}

    Child child = new Child();
    child.setChildName("Child");
      
    Parent parent = new Parent();
    parent.setParentName("Parent");
    parent.addChild(obj);

    sess.save(child);
    sess.save(parent);
    t.commit();

SQL> select * from parent1;

PARENT_ID_PARENT                  PARENTNAME
--------------------------------  -----------
8a8080e7fefc5b3100fefc5b528b0002 Parent

SQL> select * from child1;

CHILD_ID_CHILD                     CHILD_ID_PARENT                   CHILDNAME
--------------------------------   --------------------------------   ----------
8a8080e7fefc5b3100fefc5b3c670001 8a8080e7fefc5b3100fefc5b528b0002    Child