// Parent  parent is already instancied

string hql = "From Child child Where child.Parent=:parent";
IQuery q = Session.CreateQuery(hql);
q.SetEntity("parent", parent);
IList llist = qq.List();

<composite-id name="Id" class="Domaine.ParentId, Domaine">
   <key-property name="Name" type="String">
      <column name="Name" sql-type="varchar" not-null="true" />
   </key-property>
         
   <key-property name="LastName" type="String" >
      <column name="LastName" sql-type="String" not-null="true" />
   </key-property>
</composite-id>

<many-to-one name="Parent" class="Domaine.Parent, Domaine" >
   <column name="Name" sql-type="varchar" not-null="true"/>
   <column name="LastName" sql-type="varchar" not-null="true"/>
</many-to-one>

public class ParentId
{

   private string _name;
   private string _surName;

   public virtual string Name
   {
      get { return _name; }
      set { _name = value; }
   }

   public virtual string SurName
   {
      get { return _surName; }
      set { _surNAme = value; }
   }



   public override bool Equals(object obj)
   {
      ParentId otherObj = obj as ParentId ;
      if (otherObj == null)
      {
         return false;
      }

      if (otherObj.Name.Equals(this.Name)
            &&
         otherObj.SurName.Equals(this.SurName))
         {
            return true;
         }
         else
         {
            return false;
         }

   }

   public override int GetHashCode()
   {
      return Name.GetHashCode() ^ SurName.GetHashCode();
   }

}