code: public <T extends Widget & Checkable> List<T> getCheckableChildren() {
          List<Widget> children = getChildren();
          ArrayList<T> result = new ArrayList<>();
          for (Widget c : children) {
              if (c instanceof Checkable) {
                  result.add((T) c);
              }
          }
          return result;
      }
docs: Gets all Checkable widgets in the group
  @return list of Checkable widgets