Children class loop with SQL

[Ferry]

hello i have a Parent and Children classes.
But how can i make a loop with SQL trough the children class?

For the parent class is it easy

     S qSQL="SELECT * FROM  UserData.Invoice ORDER BY HDate"
     S mr=##class(%ResultSet).%New()
     D mr.Prepare(qSQL)
     D mr.Execute()
     while mr.Next() {
     W mr.GetData(3)_"<br>"

    S ID=mr.GetData(1)
    // Write children with ID

    }
    D mr.Close()

But it don't work for the children class