fix parenting of scenes

crates/bevy_scene/src/scene_spawner.rs

@@ -7,7 +7,7 @@ use bevy_ecs::{
     world::{Mut, World},
 };
 use bevy_reflect::TypeRegistryArc;
-use bevy_transform::prelude::Parent;
+use bevy_transform::{components::Children, prelude::Parent};
 use bevy_utils::{tracing::error, HashMap};
 use thiserror::Error;
 use uuid::Uuid;
@@ -268,11 +268,25 @@ impl SceneSpawner {
         for (instance_id, parent) in scenes_with_parent {
             if let Some(instance) = self.spawned_instances.get(&instance_id) {
                 for entity in instance.entity_map.values() {
+                    // Add the `Parent` component to the scene root
                     if let Some(mut entity_mut) = world.get_entity_mut(entity) {
+                        // This will filter only the scene root entity, as all other from the
+                        // scene have a parent
                         if !entity_mut.contains::<Parent>() {
                             entity_mut.insert(Parent(parent));
                         }
                     }
+                    // Add the scene root to the `Children` component on the parent
+                    if let Some(mut parent_entity) = world.get_entity_mut(parent) {
+                        if let Some(children) = parent_entity.get_mut::<Children>() {
+                            let children = &**children;
+                            let mut children = children.to_vec();
+                            children.push(entity);
+                            parent_entity.insert(Children::with(&children));
+                        } else {
+                            parent_entity.insert(Children::with(&[entity]));
+                        }
+                    }
                 }
             } else {
                 self.scenes_with_parent.push((instance_id, parent));

crates/bevy_transform/src/hierarchy/hierarchy_maintenance_system.rs

@@ -50,11 +50,9 @@ pub fn parent_update_system(
         // `children_additions`).
         if let Ok(mut new_parent_children) = children_query.get_mut(parent.0) {
             // This is the parent
-            debug_assert!(
-                !(*new_parent_children).0.contains(&entity),
-                "children already added"
-            );
-            (*new_parent_children).0.push(entity);
+            if !(*new_parent_children).0.contains(&entity) {
+                (*new_parent_children).0.push(entity);
+            }
         } else {
             // The parent doesn't have a children entity, lets add it
             children_additions