code: public function getChild($id, $relation, $idChild, $filters = null)
      {
          $parent = $this->model->find($id);
          
          if (! $parent) {
              return null;
          }
          
          if (count($filters->request->all()) > 0) {
              $child = $parent->$relation()->getRelated();
              
              $search = new Search($child, $filters, $parent->$relation());
              $this->builder = $search->getBuilder();
              
              /* Retorna os dados apenas da table/resource atual */
              $this->builder->select("{$child->getTable()}.*");
              
              /* N:N precisa add o id da outra tabela */
              if ($parent->$relation() instanceof \Illuminate\Database\Eloquent\Relations\BelongsToMany) {
                  $this->builder->where($parent->$relation()
                      ->getOtherKey(), $idChild);
              }
              
              $resource = $this->builder->get();
          } else {
              $resource = $parent->$relation()->find($idChild);
          }
          
          return $resource;
      }
docs: Return one child by id
  
  @return \Illuminate\Database\Eloquent\Model;