code: public FormModel getChild(HierarchicalFormModel formModel, String childPageName) {
          if (childPageName == null) throw new IllegalArgumentException("childPageName == null");
          if (formModel == null) throw new IllegalArgumentException("formModel == null");
  
          final FormModel[] children = formModel.getChildren();
  
          if (children == null) return null;
  
          for (int i = 0; i < children.length; i++) {
              final FormModel child = children[i];
              if (childPageName.equals(child.getId())) return child;
          }
  
          return null;
      }
docs: Returns the child of the formModel with the given page name.
  
  @param formModel     the parent model to get the child from
  @param childPageName the name of the child to retrieve
  
  @return null the child can not be found
  
  @throws IllegalArgumentException if childPageName or formModel are null