add element child list

backend/app/Element/EloquentElementRepository.php

@@ -61,6 +61,25 @@ class EloquentElementRepository implements ElementRepository
         return $elements;
     }
 
+    /**
+     * @return Element[]
+     */
+    public function findByParentElement(Element $parentElement): array
+    {
+        $models = ElementModel::where(
+            'parent_element_id',
+            $parentElement->getId(),
+        )
+            ->orderBy('id')
+            ->get();
+        $elements = [];
+        foreach ($models as $model) {
+            $elements[] = $this->toDomain($model);
+        }
+
+        return $elements;
+    }
+
     private function toDomain(ElementModel $model): Element
     {
         $set = $this->setRepo->find($model->set_id);