code: public function getSubLevel($parentId)
      {
          $data = $this->getAllNodes();
  
          $childList = collect([]);
          foreach ($data as $val) {
              if ($val->{$this->getTreeNodeParentIdName()} == $parentId) {
                  $childList->put($val->{$this->getTreeNodeIdName()}, $val);
              }
          }
  
          return $childList;
      }
docs: 获取子级（仅子代一级）.
  
  @param mixed $parentId
  
  @return array