Fix RemoveChildren command (bevyengine#6192)

# Objective

`RemoveChildren` could remove the `Parent` component from children belonging to a different parent, which breaks the hierarchy.

This change looks a little funny because I'm reusing the events to avoid needing to clone the parent's `Children`.

Co-authored-by: devil-ira <[email protected]>

crates/bevy_hierarchy/src/child_builder.rs

@@ -1,7 +1,4 @@
-use crate::{
-    prelude::{Children, Parent},
-    HierarchyEvent,
-};
+use crate::{Children, HierarchyEvent, Parent};
 use bevy_ecs::{
     bundle::Bundle,
     entity::Entity,
@@ -68,12 +65,19 @@ fn update_old_parents(world: &mut World, parent: Entity, children: &[Entity]) {
 
 fn remove_children(parent: Entity, children: &[Entity], world: &mut World) {
     let mut events: SmallVec<[HierarchyEvent; 8]> = SmallVec::new();
-    for child in children {
-        world.entity_mut(*child).remove::<Parent>();
-        events.push(HierarchyEvent::ChildRemoved {
-            child: *child,
-            parent,
-        });
+    if let Some(parent_children) = world.get::<Children>(parent) {
+        for &child in children {
+            if parent_children.contains(&child) {
+                events.push(HierarchyEvent::ChildRemoved { child, parent });
+            }
+        }
+    } else {
+        return;
+    }
+    for event in &events {
+        if let &HierarchyEvent::ChildRemoved { child, .. } = event {
+            world.entity_mut(child).remove::<Parent>();
+        }
     }
     push_events(world, events);