Return a single child property added with putChildProperty method. 
      Syntax
         
         public abstract IODataProperty getChildPropertyData (String propertyName)
       
      Parameters
         
         
            - 
               propertyName – 
               The name of the child property that should be returned.
            
 
         
       
      Returns
         The found child property or null if there is no child property with the given name.