diff --git a/whelk-core/src/main/groovy/whelk/search2/FacetTree.java b/whelk-core/src/main/groovy/whelk/search2/FacetTree.java
index 9d002d92d7..b6721d3b9c 100644
--- a/whelk-core/src/main/groovy/whelk/search2/FacetTree.java
+++ b/whelk-core/src/main/groovy/whelk/search2/FacetTree.java
@@ -35,7 +35,7 @@ public List<Map<String, Object>> sortObservationsAsTree(List<Map<String, Object>
             var children = findChildren(observation, observations);
             if (!children.isEmpty()) {
                 queue.addAll(children);
-                observation.put("children", children);
+                observation.put("_children", children);
             }
         }
         return List.copyOf(tree);